This is a good read that explains alot of the extreme hands you see when playing on-line that lead you to believe it is rigged. Personally I have seen some strange things and am not totally convinced either way, though I tend to lean toward it being on the up and up. In any event this is a good read:
http://www.pokerpages.com/articles/archives/samuel31.htm
http://www.pokerpages.com/articles/archives/samuel31.htm