Google guy matt cutts has explained how to do a site reinclusion request into google incase the site has been banned, please read it here,

First off, what’s a reinclusion request and why would you want to do one? If you’ve been experimenting with SEO, or you employ as SEO company that might be doing things outside Google’s guidelines, and your site has taken a precipitous drop recently, you may have a spam penalty. A reinclusion request asks Google to remove any potential spam penalty.
The first step is to take a long, hard look at your website. Is there hidden text, hidden links, or cloaking on your site, especially on the front page? Are there doorway pages that do a JavaScript or some other redirect to a different page? Were you trying to use some automated program to get links or scrape Google? Whatever you find that you think may have been against Google’s guidelines, correct or remove those pages.
Now where should you send a reinclusion request? This has changed in the last few months from an email address to a web form. The best location to go is http://www.google.com/support/bin/request.py . You can select “I’m a webmaster inquiring about my website” and then select “Why my site disappeared from the search results or dropped in ranking.” Click Continue, and on the page that shows up, make sure to type “Reinclusion Request” in the Subject: line of the resulting form. Upper- or lower-case doesn’t matter, but make sure you use the words “reinclusion request” in the subject line so it gets routed to the right place.

For more information visit here, http://www.mattcutts.com/blog/reinclusion-request-howto

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