We've had this thread quite a few times, and when we cut out all the speculation and "here's what _I_ would so if I were in Iraq", it usually boils down to the following:
--Some folks who only had M9s picked up AKs, primarily during the initial invasion when we had less gear than we needed
--A small minority of troops are in very independent small units with a CO who lets them get away with carrying confiscated gear. I knew _one_ EOD team (Anbar, '04) where their GySgt let some guys carry a Sterling, a few AKs, and jury-rig a PKC onto a Humvee that had no 240 for the ring-mount. GySgt himself carried a G3K. He was a THR-type gun lunatic. [edit:] I did also see one SeaBee petty officer with a underfolder AK in '04.
--Tons, tons of troops have pictures of themselves posing with confiscated gear, or even pretending to be on an op with them. I've posted my way-cool FAMAS pic here, and it's quite likely that someone on some other forum is using it as "evidence" that the FAMAS is a favorite weapon of US Marines.
That's about what we've gotten out of previous threads. The rest of it is just "this one guy said" and other such things. Long, long-running topic...