Illusory Wall -- Wizard Utility 10
You create the illusion of a contiguous wall of solid material (stone or metal, for example)... The wall blocks line of sight for all enemies (but not your allies). When any enemy moves adjacent to the wall, you can make an attack (Intelligence vs. Will) against that target; if successful, the target cannot move through the wall on its current turn...
Your opponents may not "move through" the wall. That doesn't mean they can't fall through the wall. (Falling is not a Move action. It's falling.) I would rule that the wizard's opponents get attacked by the wall's Illusion effect as soon as they're adjacent to the pit (and thus diagonally adjacent to the wall). Those who the wizard misses see the illusion for what it is. Anyone who walks into the pit (whether the illusion is affecting them or not) falls into the pit. Falling through the floor should make it very obvious that there's something fishy going on. Doubly so if the pit is filled with piranhas and sharks.
The difference here between falling and moving is easy to conceptualize. You see a great big stone wall in front of you. You don't try to walk through it, because you can't walk through walls, and it's very obviously a stone wall in front of you.
On the other hand, your buddy Jeff is wise to the ways of wizards. (He has a high Will defense.) He sees the wall and notices that it is false, and walks through, no problem. This doesn't help you much, unfortunately, because while you watched Jeff walk through the wall, he's clearly using some kind of magical ability to phase through stone. Because that wall is clearly solid stone. (The wizard is still successfully hitting you with the Int vs. Will attack.)
In short: NO, this trick does not work in 4e. Even if the wall were solid to those who it was still affecting, it wouldn't be a trap for high Perception characters, but rather for high Will characters. (Or the enemies of wizards with terrible luck of the dice.)