An encounter begins when a creature becomes aware of a possible foe
Having been thoroughly playtested largely only at low levels, the game just isn't adequately equipped to deal with the long-term consequences of utterly undetectable stealth that's possessed by some high-level characters. (Some of this playtesting is documented in the Wizards of the Coast Web column Celebrity Game Table.) To be clear, here's he Dungeon Master's Guide on Combat on Starting an Encounter:
An encounter can begin in one of three situations.
- One side becomes aware of the other and thus can act first.
- Both sides become aware of each other at the same time.
- Some, but not all, creatures on one or both sides become aware of the other side.
When you [the DM] decide that it is possible for either side to become aware of the other, use Spot checks, Listen checks, sight ranges, and so on
to determine which of the three above cases comes into play. Although it’s good to give characters some chance to detect a coming encounter, ultimately it’s you who decides when the first round begins and where each side is when it does. (22)
So it's not the sorcerer casting a spell that signals the beginning of the encounter. Instead, the encounter begins when the superstealthy sorcerer first becomes aware of his potential opponent. That's when the DM makes in vain Listen and Spot skill checks on the opponent's behalf and the sorcerer gets a surprise round.
(The Dungeon Master's Guide's examples demonstrate that if one entire side of an encounter is unaware, there's no reason for either side to make initiative checks until the first normal round of the encounter. (The aware side just goes; probably best to roll anyway if there's more than one creature on the aware side, though, as delaying during the surprise round may make a difference if the unaware side survives the surprise round!) Further, the DMG's examples demonstrate that were some members on one side aware and other members on that same side weren't aware, only the aware members make initiative checks during the surprise round. Make of all this what you will.)
At that point, because it's the sorcerer's surprise round, the sorcerer could have taken a standard action that would've guaranteed him getting the drop on his opponent and, perhaps,—were the sorcerer's initiative check result to beat his opponent's—enable the sorcerer to take a full-round action during the first normal round of combat. Rather than do that, though, the sorcerer took a chance that he would remain undetected and continued stalking his opponent. Thereafter, until the DM rules it ends, that same encounter continues.
This means that the sorcerer can—whenever he desires or the opportunity presents itself—take the full-round action that he needs to cast, for example, a disintegrate spell that's modified by the metamagic feat Silent Spell without worrying about that pesky surprise round somehow fouling up his plans. This encounter could've started, like, over 10 rounds before the sorcerer finally gets around to casting that disintegrate spell! However, that also means his opponent can take its full allotment of actions when its turn next comes up in the already-determined initiative order.
So, to be clear, on either side it's awareness—not hostilities—that determine when an encounter begins.
"That's weird. Is there an alternative?"
The Dungeon Master's Guide on One Side Aware First in its second paragraph presents an alternative that the DM can rule applies to this undetectable sorcerer:
[T]he aware side [could have] a few rounds to prepare. (If its members [like the sorcerer] see the other side off in the distance, heading their way, for example.) You [the DM] should track time in rounds at this point to determine how much the aware characters can accomplish. Once the two sides come into contact, the aware characters can take a standard action while the unaware characters do nothing. Keep in mind that if the aware characters alert the unaware side before actual contact is made, then both sides are treated as aware. (22)
This DM wouldn't apply these rules in the case of the superstealthy sorcerer, though, due to the extreme fragility of that sorcerer's position. Unless the sorcerer's been exceptionally cautious or devoted to his clandestiny significant resources, all it should take is, for example, one creature in the dungeon with an unusual sense—like the blindsight 90 ft. that's possessed by the orc warlord's trained darkmantle (Monster Manual 38) or the tremorsense 60 ft. that's possesses by the orc warlord's adept adviser's summoned thoqqua (242)—to detect the sorcerer's presence.
But all DMs aren't this DM, and another DM could reasonably rule the sorcerer so undetectable that the sorcerer who spies an opponent instead has time to prepare rather than the sorcerer's awareness triggering the encounter's beginning, the encounter actually beginning when the DM determines the two sides have "made contact." (The DMG's example that illustrates this function of the surprise mechanics isn't useful for our purposes. In it, Jozan hears some enemies orcs behind a closed door so he casts some buff spells on himself then busts in—a situation far different from our stalking sorcerer!)
That DM's decision to rule this way puts the sorcerer in the same situation posed in the question—a situation that's uncomfortable and, by the game's own measure, unresolvable absent DM intervention. That is, the game assumes making contact means I'ma gonna fight that dude right the heck now! not I'ma gonna stand here, silently cast buff spells, and wait for the perfect opportunity to launch a maximized disintegrate ray at his noggin. Good luck.
"Roll initiative!" and metagaming the response
This DM sometimes has players make initiative checks—therefore putting the players on battle footing—without any apparent threats to their PCs. In this way, the players can be pretty sure that the surprise round has already occurred, but during the surprise round either no one interacted with the PCs or the interaction that Team Antagonist attempted failed to such a degree that the PCs remain unaware of it… even though the players may know something's up. Until the threat is something they recognize, my players—who are, like most RPG players, incredibly paranoid—still have their PCs continue doing what their doing rather than, for example, getting to cover or concealment and making Hide skill checks, casting spells like see invisibility or true seeing, or—seemingly for no reason—drawing a weapon and attacking the planet while fighting defensively and making maximal use of the feat Combat Expertise.
My players have become comfortable with this because I've done this a few times to them, and they, too, have seen Team Antagonist react in the same way. An encounter will begin for Team Protagonist yet not for Team Antagonist, and Team Antagonist will remain apparently unaware of that encounter's beginning, and they'll keep on doin' what they were doin'.
The only mechanical issue with this process is that once creatures have had the opportunity to take actions after initiative checks are made, those creatures are no longer flat-footed. This means—by all counts, inexplicably—that creatures that've perceived no threats and that've not been the apparent subjects of any hostile acts from any source, after they've had a chance to act in the first round of the encounter, are no longer flat-footed while they, for example, go about their daily routine of dusting the shelves, guarding a chest, or burninating the peasants.
However, this technically oddity doesn't, like, reward indecision. While a creature that opts to let the unaware opposition continue its business will always be able to take a full-round action before the opposition can (either because the creature's initiative result beats the opposition's or because the creature delays until after the opposition takes its turns), by doing so the creature has given up the chance to take in a row both a standard action (in the surprise round) and its normal turn (by beating the opposition's initiative result). To this DM and the players in campaigns he runs, this has always seemed a fair trade.
"Can the sorcerer get two turns in a row?"
To paraphrase the question:
After the turn on which he cast the spell but before the opponent takes its turn, can the sorcerer get a second turn?
Only if the sorcerer can find a way to act twice in a row independent of how the game says that initiative functions. (A not impossible task, by the way, but certainly one that often unbalances the game!) See, when the sorcerer finishes casting the spell, the full-on, probably-visible, possibly-audible, and maybe-even-smellable spell effect comes into being, and that may tip off the opponent as to the sorcerer's whereabouts. That means, in the normal initiative order and without interruption, the typical opponent always takes its turn after the sorcerer.
It's entirely possible that the opponent will ready or delay, though, as the limited knowledge the foe's gained of the sorcerer's location may not be enough for the opponent to full attack the sorcerer or whatever as the sorcerer himself remains utterly undetectable!