The only pain is that an Ad network is a network effects business, you need a lot of publishers and a lot of advertisers for it to be worth the advertisers' penny. The big sites that have 90% of the traffic manage their own advertising because they are big enough, the small guys are just random webmasters with a few hits to spare. So what's needed is a crazy twist to this plot, um, and an accessible payment system. I have some buddies toying with this concept though, in painful stealth mode (why oh why?) though.