Welcome to WebmasterWorld Guest from 188.8.131.52
The problem with forum sites is that your visitors are focused on a specific task: reading the messages, and posting messages. And your site probably has a fixed layout so the ads are in the same place all the time, so your visitors will "tune them out." (Juggling your page so that the ads are more noticeable will just annoy your visitors.)
I am not aware of ANY CPM networks that can generate reasonable revenue for forum sites; Google AdSense is quite probably the best-performing placement you're going to find, although as noted above, you might benefit from rotating other ads from other sources.
I am somehow in the same situations, around 30 000 impressions a day and very low payments.
I think Adbrite pays after 60-90 days or something, I remember seeing something like that last time I visited them.
Anythig else that works? Any other alternatives?