The way to make it back to Google index is to simply send in the reinclusion request. This is how it is supposed to work. However, Barry on Search engine Roundtable informs about a WebmasterWorld thread which states that Google Reinclusion request forces you to ‘admit’ that you’re a spammer. 

Google’s well-known engineer Matt Cutts makes an exception to his general precedent of not talking about companies being removed from Google’s index.

Notice to search marketers: Your brand will not save you from the consequences of screwing with search engines.

One of the great fears in the search engine marketing world remains getting dropped by the big search engines. Many online businesses live and die by their Google page ranks as well as how they rate in organic searches. Sometimes Google drops the bomb and you’re done for, aren’t you? Your page rank drops, your page listings disappear and you sweat. Maybe you should try reinclusion.