I and my player are having a showdown over what the official ruling is going to be on this is. In fact I started another thread about it before I found this one.
Let me say first... and I cannot stress this enough...
IT IS NOT A POWER ISSUE!!!
I, as a DM of 8 years, do not believe that it is in the spirit of the rules to twist them around and do things that were not originally considered, like horse-bomb (in 3.0.) You could just as easily use mouse-bomb. Should I allow it just because it is not overpowered?
Furthermore, if it is the case that creators of the game rule that you can do this, like they have with armor spikes, then this raises all kinds of interesting technical problems for the rules system. For instance, can a tiefling with a vestigial arm take improved multiweapon fighting (just because she has an extra "hand" and then multiweapon fight with two shortswords and her foot? Well... I would say that if the two weapon fighting thing is kosher, because of the foot counting as an "offhand," then the logical extension of that would be this and lots and lots of other silly tactics.
Perhaps in your game this is preferable. I myself have been DM in games where I encouraged and even rewarded silly tactics, but I do not think that this should be the norm (especially in official organized play.)
Another problem... what do you do with monsters who have lots of limbs? If a cestus can't be used with a greatsword, because you already used it (the arm that is, just like you can't two weapon fight with one dagger by quick drawing it out of your other hand, because you already used it: the weapon,) then does my hypothetical tiefling with multiattack get two swords and two feet? What about a marilith? Can she get a tail and a head-butt in? What about the tieflings head-butt?
My friend (who has been playing longer than I have been DMing) says that the answer to my last string of questions would be ‘of course not!’ because you can only attack with unarmed strike as if it were a single weapon, so the marilith could get an extra attack if she had improved unarmed strike, but no more than one. An atach could get an extra unarmed strike in with his foot, but not both feet and his head.
This makes perfect sense in the case of monsters, but unfortunately for me, long winded and rules lawyery as I am, it raises another problem. What if I am an unarmed fighter who uses his fists, like a boxer? I can't do this now because I can only attack with one unarmed strike. The unarmed strike could be my primary weapon, gaining my full strength bonus to damage and extra attacks by virtue of a high base attack bonus, but I must be wielding a real weapon in my other hand. I am no longer allowed to duel wield my own two hands. This is in my mind a disastrous implication because it limits the PCs in a more significant and relevant way than not allowing them to two weapon fight with their foot and a greatsword. What if the ranger gets disarmed? Is he completely bum!@#$ing useless now?
It is perfectly OK with me if someone in a position of authority to make this call for errata purposes says on a case by case basis “Yes, you can do this. No you can’t do that.” Please make me not confused anymore.