kreynolds said:
Nope. Not good enough. Cleave over-rides that.
So you can Cleave into someone who has full cover? Who's out of your threatened range, as long as he's in the immediate vicinity of the fellow you dropped?
I don't think so.
Cleave lets you take a melee attack with the same modifiers as the original attack. Clearly, you can't use that melee attack on anyone you couldn't normally use a melee attack on. Reasonably, you could also rule that you couldn't use the cleave attack on anyone who wasn't a valid target for the original attack. You could also rule it the other way, and say that you can cleave into someone who hasn't provoked an AoO.
Thing is, that leads to lame results, for instance:
Fighter (combat reflexes & great cleave, among other things) is locked in melee with a powerful enemy. The fighter's caster buddy, a Cleric, happens to have Summon Monster 3 prepared. Now, the Cleric might want to use searing light - he'll be making a ranged touch attack at -4 to hit the baddie, and only inflicting a few dice of damage. Or, he /could/ use his summoning spell to call up d4+1 celestial badgers, and, since he speaks celestial, command them to attack, not the baddie, who's AC is way to high for the little buggers to hit, but the fighter. Why? Because badgers are tiny creatures and will provoke an AoO from the fighter. The figher effortlessly hit and kill each attacking badger before they can actually hit him, and Cleave into the main baddie, at his full BAB, each time.
Now, aside from this tactic being absurdly potent (5 full-BAB attacks from a greatsword wielding mid-level fighter probably equating to something like 10d6+50 damage, vs 4d6 for an 8th level searing light), it's just plain lame. It is, simply, an undesireable result of a an overly litteral ruling, and easily prevented with a more thoughtful, less obvious, but still basicly legal ruling.
Use whichever ruling you like, but expect your players to react to it as best suits thier interest.