I think so long as the USB microphone is supported as an audio device (meaning it shows up in the 'Sound' system pref as a legitimate input device) it'll work with iChat AV. My PC headset works plugged into the headphone and microphone ports on the back of the computer and my bluetooth headset works fine as a microphone for iChat and skype.
Logitech and Plantronics are two brands that I've used as headsets and found them to be comfortable with good mikes (never used with iChat AV tho').