Things are so awful in Obama's America that even the Man of Steel can't put up with it anymore. http://www.foxnews.com/entertainment/2011/04/28/superman-renounces-citizenship-00th-issue/
He was legally adopted by Jonathon and Martha Kent, making him a US citizen, so long as they were both US citizens.
Superman couldn't put up with Obomba beating up white guys in McDonald's restaurants. It's counter to his belief system. Only he, Superman, should beat them up.
Superman was jailed by Eric Holder for trying to apprehend the two blacks girls beating on the white transvestite, and Al Sharpton and Gloria Allred are representing the girls in a civil lawsuit against him, as I just saw their press conference on FauxNews. That was the last straw for ole' Supe.
If Superman is so weak that he can now be jailed, I don't think it's terribly impactful that America has lost him. Put him on the terrorist watch list and keep him locked up indefinitely. His renouncing US citizenship just makes holding him forever without a trial that much easier.
It's more symbolic than anything. Supe mailed it in after the '80s. The sad thing is now we're down to only one true American Superhero: Yeah, I'm completely in favor of imprisoning him in the Phantom Zone. That'll teach him to screw with the Red, White, and Blue!