Sorry, I'm late on the draw on this one, but it's been a busy day for me. Just in case you haven't already heard, Matt Cutts announced that some webmasters who use Google sitemaps may get notified of certain penalties they are receiving on their sites.
If you are a big time black hat spammer, you won't get notified, but hey, you already know what you are doing "wrong", right? If you have an otherwise legitimate site, however, you may be given a broad statement that you aren't meeting their guidelines, and that after you figure out the problem, you can ask for reinclusion. You won't be told exactly what the problem is.
It seems as though this is mostly concentrating on some of the "simpler" techniques, such as hidden text, so if you get the message, that's where I'd look first.
Personally, I wouldn't bother participating with my own sites. If you're an SEO, you know if you are using tactics such as hidden text or not, and you don't need Google to tell you why you are penalized.
However, if you are not an SEO, or you have a client with a penalized site, then it might be worth it.
Kudos to Google for taking this step, even though it is just a small step. Of greater use, would be the ability to get feedback for problems such as canonical issues, redirect goofs, etc. Maybe someday...