We knew it was coming — RSS Adsense — annnounced today via the Google blog :

Enter AdSense for feeds, launching today in beta. The idea is simple: advertisers have their ads placed in the most appropriate feed articles; publishers are paid for their original content; readers see relevant advertising - and in the long run, more quality feeds to choose from.

Furthermore, they list a page with best practices for advertising inside RSS feeds with their own set of suggested guidelines.

Publishers “apply here” — no clue what their criteria is for acceptance to this public beta, nor how long it takes to get accepted or rejected. I signed up and am waiting to see what happens next. The clock is ticking.