Tribal fusion is good if you have a lot of page views but few clicks(they just do CPM ads, but are pretty much top dog in the space, so they can get better rates), but you need a lot of traffic to get in.

You need about 5K uniques per day to get in, although that number varies by category...for example I hear that if you have a game website, you'll need 20K uniques per day to get in.

I just needed CPM since I get a lot of page views per user, average is only ~6 pages, but that number is distorted by all the one page wonders from when the site hits some social network. The actual users who come to the site from niche specific sources, do about 20 pages per hit.