I know bloggers who make an entire living from Adsense, but they really have to work at it, blogging on topics that have good ad linking. Much harder to do that well with forums, I think. I've chosen not to run ads on forums, but have received very good donation rates.
I experimented with Adsense on the thousands of pages I have that are not forums or themselves selling retail product, and eventually reduced the ads to just three pages that performed well. Originally two, but expanded to three to increase ad exposure. A grand total of $1388.44 since late 2004
