They are building a network of bloggers to participate. Marketers can browse bloggers in the network and WP’s sales team will sell ads for you. WP and the blogger splits the revenue (no mention of what the split is).
The additional attraction to joining the network for bloggers however is that those participating get a rotating link to their blog on the WP’s homepage.
They seem to be looking for blogs in the fields of Technology, Business, Health, Automotive and Travel at this point.
It’s definitely an interesting system that goes a little further than most other blog advertising systems that I’ve seen (in that it not only makes you money but potentially sends you traffic) and one I’ll be keen to check out.