In their most recent blog entry, AdSense offers up an AdSense heat map specifically for forums, likely as a result of their forum optimization tests that some publishers were participating in this summer.
I would be curious to see how this breaks down across forum styles, if they release this information. I found with phpBB that the right column, while higher CTR than some placements, was definitely not as good as immediately following the first post of a thread. But even then, forum CTR is notoriously low in comparison to a non-forum page.
However, forums that manage to get a lot of CPM ads can do quite well revenue wise. So be sure to enable image ads, but also being sure to use one of the ad unit sizes that supports image ads. These are:
- Leaderboard (728 x 90)
- Banner (468 x 60)
- Skyscraper (120×600)
- Wide Skyscraper (160×600)
- Medium Rectangle (300 x 250)
The other ad unit sizes will not be able to take advantage of the full range of CPM ads, since while other ad units can display the text CPM ads, you would be missing out on all the image CPM ads.