Joel Stein has a great article on this. (Thanks to Rachael for alerting me to it.) I've thought long and hard myself about what I'd say if I were in a public forum and someone tried to corner me on this. (Because of course to just say "no" and move on would be bad.) The best I've got so far is to say, "Not all of them, do you?" and then get the person to deny support for torturers, etc.
(Oh, for you purists out there, I do support some of the troops--so my suggested answer isn't completely duplicitous. E.g. purely medical personnel, or translators, etc. OK, my response is still pretty duplicitous...)