According to Google, here is the best ad placement for forums:
Quote:
Certain locations tend to be more successful than others. This "heat map" illustrates these ideal placements on a sample page layout. The colors fade from dark orange (strongest performance) to light yellow (weakest performance). All other things being equal, ad placements above the fold tend to perform better than those below the fold. Ads placed near rich content and navigational aids usually do well because users are focused on those areas of a page.
Best ad placement for forums according to Google
Linear Mode
