In the past, if your site was banned and delisted from the index. The way back into the Google search index was to go to google.com/support/bin/request.py and select the reinclusion request option.
That option has been moved to Google Webmaster Central.
Adam Lasnik of Google said;
Putting the reinclusion request within Sitemaps has actually been quite helpful in a lot of ways; in particular, it's prompted Webmasters to do some self-troubleshooting with our new tools, often highlighting simple steps they can take to fix problems that have prevented their sites from getting crawled, and saving them from the futility of filing a reinclusion request when they weren't "unincluded" intentionally, though it hasn't as of yet prevented Google employees from writing run-on sentences. Unfortunately.
Forum discussion at WebmasterWorld.
This article was written the night before, and scheduled to go live at this time.