As I understand it, by RAI, there's no way out of Ravenloft other than the Dark Powers letting you out, such as via a portal or the mists just transporting you out. I'm wondering if it's possible to get out via RAW without the Dark Powers' permission. Can you Iron Heart Surge the mists, for example? I'm looking for a solution that's within RAW in either 3.5 or Pathfinder, even if it's cheesier than Pun-Pun.
RAI compliant: Major Artifacts can beat Ravenloft's travel restrictions. (Ravenloft PHB p.123):
At the DM's discretion, the abilities and spell effects created by major artifacts may remain unchanged in Ravenloft. Major artifacts may even possess the power to pass through closed domain borders or burst free of Ravenloft's clutches entirely. However, evil, intelligent major artifacts may not want to leave.
Planar Bubble (Sor/Wiz7, Clr7, SpC; or Planar Shepherd class feature):
This makes your surroundings obey the planar traits of your native plane instead of wherever you actually are. So obtain a creature that's native to somewhere other than Ravenloft (via a calling spell if you yourself aren't), cast Planar Bubble on it, and then leave via ordinary Plane Shift. As an abjuration spell that isn't about banishment, Ravenloft doesn't by default corrupt the casting of Planar Bubble; and once the Planar Bubble is active, Ravenloft can't mess with your subsequent spells.
Precipitate Complete Breach (Sor/Wiz9, Planar Handbook; or Anarchic Initiate class feature):
As this is a teleportation spell, it is subject to Ravenloft's rule that "no spell can transport anyone or anything across a closed domain border or out of Ravenloft" (Ravenloft PHB p.109). So this only works under the interpretation that the spell isn't transporting you; the spell is making two planes coexistent even if they weren't connected before, and then the transport is done by ordinary walking.
Gatecrasher 3 (PrC, Manual of the Planes) or Silver Key 10 (PrC, Dragonmarked):
If there are portals out of Ravenloft, and if you can find one, then these classes let you use the portal without the Dark Powers' permission. Gatecrasher in particular can explicitly beat portals that were sealed even by deities.
Find The Path (Clr6) can guide you to such a portal, "provided you know what you're looking for" (Ravenloft PHB p.112). Obtaining a precise enough rumor to start searching, however, may still be difficult.
While you should never cast Wish in Ravenloft (it's always either twisted or fails entirely even if you wish for something that's safe under the core rules), someone who's already outside could use the "transport travelers [...] regardless of local conditions" mode to rescue other people.