Thanks for sharing this info with us. Like you say, ads on Tumblr blogs are not new - but until now, I think they were against the terms and conditions and could be removed. There's also a lot of subtle affiliate advertising in posts by large blogs already.

I imagine that Tumblr will take a large percentage of any ad profits, and that it'll only be useful for blogs with very high pageviews - but this system sounds very convenient for users. Taking ads into your own hands might reap higher rewards, but an integrated ad system seems more transparent and accessible.