i saw someone blatantly cheating on a computer science test, once. no books or calculaters were allowed, yet they had their books on the floor, open, and were flipping through them. they also had programmable graphing calculators beside them, open, and were punching buttons on them. they later asked a TA for clarification on a question, and the TA did nothing!
that annoyed me so much that i reported them to the teacher. at the next test, the teacher wanted me to point them out to him. if exposing cheaters (atleast at the university level) requires you risk being seen by the class as a "tattle tale", it's not worth it.
of course, cheating on a test and taking action on a message board are two diff. things.