I stand corrected; Hague Convention and not Geneva. In any case, I think we're bound by it, regardless of whether or not others are signatories.
Sorry, too many years in school getting corrected
The actual articles state:
"Article 2 The provisions contained in the Regulations mentioned in Article 1 are only binding on the Contracting Powers, in case of war between two or more of them.
These provisions shall cease to be binding from the time when, in a war between Contracting Powers, a non-Contracting Power joins one of the belligerents." (http://avalon.law.yale.edu/19th_century/hague02.asp)
Since we're fighting a non signatory group (not even a nation), the conventions don't technically apply (lord knows they don't abide by them). So we could in fact use the nasty stuff and have legal loophole to do so, the world media would crucify us and it would take a political leader willing to sacrifice his/her career to do it.
I just always found it odd that we were abiding by rules made over 100 years ago by another country (Tsar Nicholas I held the convention) for a group that no longer exists (league of nations).