To have the site re-included in the index, you need to submit a reconsideration request to Google.
Before you submit the request you should clean up the problem that led to the ban.
You should ensure that your site follows the Google Webmaster best practices.
You should be repentant in your reconsideration request, admit what you did, explain why you did it and that you won't do it again.
A reconsideration request is processed by a human, so there are no shortcuts, you have to either do the work to clean it up or you won't have a hope of being successful.
Go here
http://support.google.com/webmasters...n&answer=35843
Once reconsideration requests were a one sided affair and you had no idea if the request had been looked at let alone processed. Now Google has a messaging system so you will get feedback about your request one way or the other.
The most important thing you can learn from this is that there are no shortcuts anymore. I know there are successful people using XRumer and SEONuke to build backlinks, however more often than not this will catch up with you.
In my own testing Google is getting much better at analyzing a sites link profile, unless it looks natural to Google you run the risk of trouble.
The same goes for spun content, Google is actively working on hardcore AI to mimic human reviewers (yes Google does have manual reviewers of sites). In 2012 it's going to get much harder to avoid being penalized as Google ramps up it's use of algorithms like Panda to weed out shallow, poor quality and spam sites.