For all of you who don't believe its possible, and for everyone who is willing to take the risk. I've posted about this before but not a lot of people think its possible. Just remember it is not guaranteed that you'll get citizenship, you might just get a green card, or you might be put in deportation proceedings. This is the way it works, you need to find a recruiter that is willing to let you enlist while being undocumented, you're not going to lie to the recruiter, but not every recruiter will let you enlist. After your first day of active duty you would be able to apply for citizenship, but USCIS can deny your application, so like I said there are no guarantees.
Here are the sources:
http://www.metroactive.com/metro/09....news-0738.html
http://www.ilw.com/articles/2009,1124-stock.pdf