Wow, Zappo, that IS clever.
What you should also do is have invisible, incorporeal silent creatures immune to force effects and ghost touch weapons who heal the bad guy each round. That way, if any of your players DID bother to do the math and play their characters effectively, you could just undo any damage they take, too!
Another fun thing is to just make the enemy wizard and leave off his magical items, and then when your heroes cast fireball, you just decide right on the spot that he's got a ring of fire resistance, and when they switch to acid, he's got an amulet of acid resistance, and so forth. That way, no one in the party ever gets to do anything! That's brilliant!
*Cough*. Or you could provide enough clues to let the player know what their characters would know -- ie, the AC of the creature. Because, as I previously mentioned, "I hit, I miss, I hit" doesn't give much flavor, but in real life, the melee guy will know whether he's missing because of his enemy's dodges or because of his enemy's armor, and he'll know within the first few rounds how much he's missing by. You know, actually let them effectively use the feat that they purchased the one or two times in their life that it's actually useful? And if it's abused, in the "Take five minutes to determine how much to Power Attack" case, you deal with it on an individual basis?
Nah, that'd let the players potentially win. Can't have that.
No offense, but that just really strikes a nerve with me. Maybe some warrior would be smart enough to drop his shield or go into Expertise mode specifically to screw up the other guy's math -- but you're describing it as the DM, doing it to screw over your players. That's just not cool.
-Tacky