What are the chances that the most talented US runners are also the most morally suspect? You would think that one of Bairu, Rohat, Teg, Solinsky, Ritz, Goucher, Rupp, Webb, Jager, Flanagan, Begley, would stand up and say, "I'm not taking this sh it" and go find a different situation. You would also think someone would leak it. A wife, a gf, a drunk idiot.
In a group training environment, its going to be everyone or no one. I'm sure they're too close and are up in each other's business too much to be able to dope without their teammates finding out. I would be more suspect of groups consisting of 2 people busting fast times, or an individual. Its much easier to convince one other guy to do it with you, or to go at it alone, then to convince a whole group that they need to do it.
So as far as our US guys are concerned, I think the Portland crew, at least, is clean. I would guess Rowbury and the Cook crew is clean. I don't really have an opinion about anyone else.