According to the Huffington Post,
In "Action Comics #900," Superman will renounce his American citizenship, rejecting the international notion that his actions are part of US policy. The shift comes after a personal visit to Iran in support of protestors leads President Ahmadinejad to believe America was declaring war against the government in Tehran.
By rejecting his citizenship, Superman will now work on a grander international scale, because, as he says, "truth, justice and the American way... it's not enough anymore."
That's the official story. I blame the damned birthers. Just because Clark Kent couldn't produce a "long form" birth certificate...