Specific example - If a rogue successfully hides from one opponent can he then sneak attack that opponent? Or can the opponents allies use a free action to yell out, "Hey, the annoying guy with the crossbow is just around the corner, keep an eye out for him!" Thus denying the rogue combat advantage?
I believe that being hidden is on a per-enemy basis. I don't think this is explicitly stated anywhere. The Rules Compendium (page 152) says "Opposed Check: Against the passive Perception of each target creature present." My assumption is that if it was all or nothing, that would read more like "Opposed Check: against the highest passive Perception of all target creatures present." However, this is definitely an assumption.
There's definitely no provision for letting someone know where a hidden target is. A hidden target has invisibility, again as per page 152. The rules for invisibility, on page 221, specifically say you have to target a square in order to attack an invisible creature. Hidden creatures don't automatically give away the square they're in; other invisible creatures (e.g., gnomes) do. But even if you know what square the target is in, you don't have the exact location. All normal penalties apply. Likewise, the invisible creature's targets grant combat advantage.
Come to think of it, let me extend that point a bit. If you think that someone who can see an otherwise invisible creature can point them out to his allies, you're opening a can of worms with regard to blindsight, tremorsight, and the like. The same logic would apply to those abilities, so you'd only need one PC with blindsight to let the whole party wander around without lights during combat.
This ruling is clearer now.
Being hidden is still on a per-individual basis, and so the basic answer is still you have combat advantage against anyone whose passive Perception you beat.
But there are now rulings about pointing out hidden creatures to others. I'm not sure exactly when it changed, but the Compendium glossary on Perception now says:
If a creature finds a hidden creature, it might point the hidden creature out to others, resulting in them knowing its location.
The rules still don't say what kind of action it is to point out a hidden creature, but speaking is a free action that can be taken on other creatures' turns.
This doesn't remove the hidden condition (so you still have combat advantage), but now everybody knows what square you're in (and so will target that square with attacks). You also automatically lose the hidden condition against anyone who attempts to enter your square, which is easier when they know what square you're in.
In essence, creatures that spot you can point your location out to their friends, who can then easily find you on their turns. IE, your location is known once at least one enemy knows your position.
[Note: Stealth is normally rolled against the passive Perception of any creature being hidden from. Making an active Perception check to find a hidden creature is a minor action.]
The Rules of Hidden Club should be useful in clarifying further confusion.
Combat Advantage is enemy specific. So even if his friend says, "Hey, the Rogue's right behind you!" he might not have the time or state of mind to look for the Rogue (who might already be in his blind spot).
I'd say, in order to keep it from completely negating the Rogue's advantage, if the enemy can't give a reasonably good description in the span of a free action (about 5 seconds), he can't give enough away to ruin the chance. Also take into consideration WHERE the Rogue is hiding. I've had people tell me exactly where to look and STILL not seen what they were referring to. I'd imagine it's even worse in combat!
I'd go with giving the enemy an active perception roll as a minor rather than a standard (with appropriate DC). Minor since he knows what to look for, rather than looking around for "Someone sneaking up on me" but rather "The Human Rogue".