There is an interesting thread going on at the WebmasterWorld forum looking at whether or not Google favors forums in search results. Matt Cutts tells me that he can neither confirm nor deny this. The thread is started by someone with the handle "bouncybunny," who posts:
Once it was directories, then blogs, now forums… maybe.
I have no empirical evidence for this, but it seems to me that Google is increasingly returning results from forum posts…