What effect are you going for? In other words: why do you want called shots?
If you want your players to be able to describe how they're attacking, well, let them. "I hack at his neck!" + an attack roll isn't a called shot; it's a fun description. If the attack succeeds (normal roll that beats AC) but doesn't kill the target (normal damage that doesn't exceed HP), describe how the foe barely deflects the lethal blow but keeps on fighting. If the damage exceeds HP, go ahead and describe the foe's head coming clean off.
Fun!
If the player has been listening to your combat descriptions and comes up with a shot that would be particularly effective (DM: the blackguard leans over to deliver a coup de grace to your fallen comrade! Player: He's leaning over? Called shot to the back of his neck!) then go ahead and reward the player with a circumstance bonus on the attack roll. That's what circumstance bonuses are
for.
If you want your players to be able to achieve specific game effects, well D&D already handles that. If the intent is to hamper the foe's ability to attack ("Called shot to his eyes!"), then have the player make an "Aid Another" check, aiding a buddy's AC. That's already in the rules. If the intent is to disarm, that's in the rules too. If it's meant to cripple, there are feats for that (Hamstring and other sneak attack feats, or the feats from PHB II that force the target to make saves or suffer Sickened, Stunned, or other conditions).
And, again, you can always impose circumstance
penalties on the bad guys as a way to reward called shots:
DM: The rogue scampers up the wall, fleeing the scene of the crime!
Player: No way is he getting away! Called shot to his hands!
DM: Okay that's a -6 circumstance penalty to your attack, but if you hit and do damage he'll have the same penalty to his Climb check.
Point is, there are plenty of rules that can cover attacks that aren't vanilla strikes. Use them! And if all the players want is more detail, well, you don't need rules for that.
-z