Straight from the FAQ (if you value the information it contains):
In a combat, Bob the fighter is unconscious from wounds, and Grog the orc moves into Bob’s square. A friendly cleric heals Bob so that Bob regains consciousness. So the orc and prone Bob are now occupying the same square. Now what happens? The rules say that Bob and Grog can’t be in the same square, right? So, what happens? Is Grog somehow prevented from entering Bob’s square? Does Grog have to leave when Bob wakes up?
You can enter another creature’s square and even stop there if the creature is helpless, or if that creature is much bigger or smaller than you are (see Moving Through a Square in Chapter 8 of the Player’s Handbook).
Assume that Bob is prone (no rule says you’re prone when you lose consciousness, but common sense demands that it be so), and that he can remain in Grog’s square so long as he
remains prone. If he gets up, he must leave Grog’s square first, which provokes an attack of opportunity from Grog.