In issue #900 of Action Comics published on 27 April 2011, Superman discusses his intention to renounce his American citizenship.
This has generated considerable discussion - see NPR, Reuters, The Weekly Standard, io9, Wired, etc
Associated Press report
Clark Kent may have a fake birth certificate and fake adoption papers arranged by Jonathan and Martha Kent. Surely Clark Kent's real persona Kal-El would not have an equivalent birth certificate from Krypton and then a naturalization certificate and nor would his alter ego Superman.
Questions need to be asked about how Superman, as an alien, became a naturalized US citizen. As a super human of non-Earth origin with special abilities, he must be above politics.