If you’ve been banned from Google for whatever reason you want to do a reinclusion request. I’ve previously used Matt Cutt’s tutorial, however it seems Google has changed the way one does this. The link Matt mention is no longer there.
The new way is to do it here, from “My Sites”. Yes, that means you will have to have registed your site there first. You’ll also have to agree that your “site has violated Google’s quality guidelines” and that you’ve made changes. In my case I don’t know why I’ve been banned, but I just agreed to it anyway.
I guess when you have all your sites registered one place it makes it easier for Google to keep track on you ;) .. so if you’ve got one banned site maybe the other looses some PR or something? (just speculating, or course).