I'm cartain that a US citizen cannot be deported from the US

but they can be sent to Gitmo.

The above answer is correct. Once you earn US Citizenship, you cannot be deported, you just go to jail..

A US citizen can be deported in two ways: 1. ICE doesn't bother to check or refuses to recognize that a person is a US citizen. This has happened several times including most famously to Peter Guzman, a Los Angeles native who is mentally handicapped and was in jail for a misdemeanor trespass. When his time in jail ended, jail employees turned him over to ICE who weren't interested in his citizenship and packed him off to Tijuana. He spent about three months living homeless in Mexico, a task made even harder by the fact that he spoke almost no Spanish. Finally, starving and sick, he wandered into a border post where somebody believed him and got in touch with his family. ACLU had sued the government on his behalf but a Federal judge had refused to require ICE or the LA sheriff's department to look for him. There are about 20 other similar cases that you can find with a Google search. The problem is that while the law says that ICE must prove that anybody they want to deport is not an American citizen in fact Immigration judges (who are ICE employees, not federal judges) normally assume that anybody before them is an illegal immigrant unless they show really good documentation. Most of us don't have documentation that is up to their standards (original copies of birth certificates, passports, etc.) on us if we are arrested. Since deportation proceedings can be over very quickly, somebody who doesn't have strong family ties, money to hire a good attorney (you don't have the right to a court-appointed attorney in immigration court) and so forth can be thrown out before establishing their citizenship. 2. A naturalized US citizen (somebody who was born with another citizenship but immigrated here and got their citizenship) can be deported if they lied on their applications either for their original immigrant visa or naturalization. This was the case for Ivan Demjanjuk, an immigrant from Lithuania, who came here after WWII. He said he had never been in the Nazi party or the SS, but later on they found out he had been a guard at an SS death camp, Treblinka, where over one million Jews and others were killed. He was suspected of being the guy who actually dumped the poison into the gas chambers, but it was later shown that he was "just" a guard. He was deported to Israel, where he was tried but eventually not imprisoned.

Yes, in extreme cases, naturalized citizens can have their citizenship stripped and be deported. http://www.dawn.com/2005/01/06/top11.htm