i tried it on couple of sites in totally 2 different niches but the top and sidebar ads performed way better than footer ads for me
The footer ads wont perform well because of one simple reason: they aren't in view. I always advise people to make sure their ads are showing when you just go on the site. If you have to scroll down to see the ad then you wont get many clicks. Try putting the blocks in primary positions like sidebars and headers.
Footer ads are nice if you are sure, that the visitor has to and will scroll down. Golden rule: Only have one ad visible on screen at any given time. Use footer ads only on those pages, that are long enough to scroll the heading banner out of sight.
I'd say on a forum it might be alright. I don't have any channel data to back it up but my users generally go to the bottom of the page to check forums and stats. I think I'll add a second channel to my bottom ad to actually find out if its doing anything.