The service should now be public: https://www.google.com/adsense/aff-settings

I wrote a small review at: http://googlesystem.blogspot.com/2008/08/google-tests-adsense-for-feeds.html

Is this really new? Maybe they expanded it?

Working very well on Zorgloob feeds...

I made a tuto (french) to keep the old feedburner feed adress with ads.

http://www.zorgloob.com/2008/08/publicits-google-pour-les-flux-cest.asp

Luka:
I think it's a bad idea to burn your feed once again and use the new feed as a source in the old FeedBurner. You should send an email to adsense-suport-affgoogle.com and request the migration of your feeds to a Google account. This way, Google will automatically redirect the feed to the new address. At some point, you won't be able to access the account from feedburner.com, so it's better to migrate to a Google account.

There is a bad point. With feedburner, the account wasn't related to any Google account when we – writers of Zorgloob – can access the feedburner account in order to see statistics.
Now, if it is linked to a Google account, it will become impossible :-/
Another day, another shit from Google.

Ionut : Why burning twice is (or could be) bad ?

- you need to choose a different URL for the feed. if you had feeds.feedburner.com/name, you can't select feedproxy.google.com/name (this is reserved).

- at some point the old feedburner site will be closed and all the feeds that haven't migrated will be redirected to the feedproxy.google.com address, so you may no longer have control over the initial address.

- the updating delay accumulates, so the new feed might be updated less frequently.

I didn't try it, but there's an option to transfer feeds to a new account. The official migration option is by email.

Is this official and public just now (since 15th Aug)?

the good thing about feeds is that you can probably get/make a tweaked reader to block javascript, iframes and whatever-have-you. that should get rid of most of the unnecessary extras...till google figures how to get around that too :D

Most feed readers already block JavaScript, iframes and objects. AdSense for Feeds uses images to display the text/image ads and these can be easily blocked with AdBlock Plus or similar software.