Improvising a bit, I would say that sleeping in light armor has no consequences (at least for a few nights), but sleeping in heavy armor requires an endurance check: failure means you start the next day with one less healing surge.
Realistically speaking though, it's probably easier to sleep in Chain armor than even leather. Leather armor isn't like a leather jacket, it's boiled leather that's very thick and very stiff. Chain at least is flexible and has padding underneath...I would think you could get away with one night's sleep wearing it.
Of course, then we would need rules for individual armor types...
Probability of a midnight ambush would depend on how well the party prepares... If they hole up pretty well, hide their position, keep quiet and setup a guard rotation, their chances are low... However an adventuring party that decides to sleep in the middle of a corridor without any guard or preparation deserves what they get!
Part of the problem with that is that sometimes the "party" makes decisions that some members don't like. For example, in our last adventure the rest of my group decided that we were taking an extended rest inside the dungeon using some back room. I repeatedly voiced my concerns and said we should either press on, or leave the dungeon. They didn't agree, so we rested there. Under your rules I would've been
screwed when we got ambushed (which we did right as the adventure ended. Our next session will actually be playing the ambush encounter) because I would've had to take my armor off. Either that, or I could've passed up the Extended rest entirely...but that's not a great choice either.
Edit (after reading Doctor Proctor's post): I haven't yet tried Paragon or Epic play, but how about making the ambush a "wolfpack" type of encounter with 4 minions and 3 skirmishers, all of them of one or two levels below the party?
Even if you went a couple levels down, you still run into the same problem. Certain classes will be almost guaranteed hits, and others will fight quite effectively. If you went, say, 4 levels down, the monsters should have 4 less to hit...give or take. So that level 16 Abyssal Ghoul with the +21? That would become a +17. It's still basically a guaranteed hit on the non-DEX PC's (2+17=19, against AC 18. They'd only miss on a 1.) but it does compare more favorably with the high DEX PC's as they have an AC of 24 (hitting on a 7, as opposed to a 3).
I suppose if you could fine enemies that weren't really high damaging such that they wouldn't kill the PC's in 3 or 4 rounds of guaranteed hits, then it might work. However, if you're throwing that easy of an encounter at them, then what's the point? They're resting after a day of fighting level 16 and up monsters and then they get ambushed by level 12 or lower pushovers? It just doesn't really make a lot of sense. You're basically going to either have an encounter that sticks out like a sore thumb, or one that will most likely kill one or more players, if not result in a TPK.
(If you're going for something special like them getting ambushed at an inn, then the weak enemies might work. Basically, whoever was trying to kill them should've sent better assassins.

Out in the field or in a dungeon though, it really sticks out.)
Edit: On a side note, if anyone is going to run or is in a campaign that has rules about sleeping in armor like this, I suggest you look at the Summoned armor mod in the Adventurer's Vault (p 53). You don't get access to it until level 6 (+2 enhancement minimum), but it costs the same as regular magic armor so it's cheap and easy to keep up.
Summoned Armor Level 6+
One need never worry about being caught unarmored while
possessing this extraordinary armor.
Lvl 6 +2 1,800 gp Lvl 21 +5 225,000 gp
Lvl 11 +3 9,000 gp Lvl 26 +6 1,125,000 gp
Lvl 16 +4 45,000 gp
Armor: Any
Enhancement: AC
Power (At-Will): Minor Action. You banish this armor
to a secure extradimensional location. At any point in
the future, unless you are wearing armor, you can use
another minor action to recall the armor. The armor
appears on you as though you had donned it normally.
So, one thing you could always try is to keep a set of Summoned Armor that's a little behind your "good" armor. If you ever find yourself surprised and without your armor, then you just need a minor action to don it. Even if it's 1 or 2 points behind your normal armor in terms of enhancement bonus, it's better than losing ALL of your AC bonus. If your DM was nice enough, he might even be willing to houserule up a set of +1 Summoned Armor.
