Quick Opinion
I don't think the monk should any penalty to attacks. According to the rules, monks don't need free hands to make attacks; they can be carrying two cages with chickens in them and have no penalty. Thus, I'd think losing one arm or even both would not affect their combat abilities (skills and the such, sure).
You can be like that shaolin monk in Shaolin Soccer who attack everything with his head
I'd love to DM a PC with that concept.
Monks bring new meaning to being able win with both their hands tied behind their back (well.. besides wizards/sorc with still spell)
-2 to hit (or something along these lines).
This due to the fact that you might be using your whole body you do have less "options" to choose from and that would make it harder to fool an enemy and hit him so to say.
I don't think the monk should any penalty to attacks. According to the rules, monks don't need free hands to make attacks; they can be carrying two cages with chickens in them and have no penalty. Thus, I'd think losing one arm or even both would not affect their combat abilities (skills and the such, sure).
You can be like that shaolin monk in Shaolin Soccer who attack everything with his head

Monks bring new meaning to being able win with both their hands tied behind their back (well.. besides wizards/sorc with still spell)