Superman isn't an American. He's a known alien whose only official place of residence is an underground bunker in Antartica. While he does spend a lot of time in Metropolis, he's certainly got a good track record of saves all around the world. He's saved planes with bad engines in Argentina, put out brush fires in Africa, plugged failing dams in Korea, and rescued stranded subs in Russia. He's shown himself to be a watchman for the world.
Then there's Clark Kent. Smallville just had it's series finale, and I was a bit distressed at the choices the program made right at the end. Basically, they settled into the classic notion that Clark Kent exists mainly as a disguise for Superman.
It's a shame, because the series for most of it's run did a good job of making you understand that Clark Kent was the real person at the heart of Superman. Clark doesn't exist to give Superman a place to hide when he's tired. Clark exists because that was the only name he knew for the first two decades of his life. Clark Kent isn't a secret identity... he's an ordinary man with mid-western values who just happens to have a laundry list of superpowers he's mildly embarassed to possess. And, despite some false information on his birth certificate, Clark is 100% American.
The proof lies in his choice of career. Clark's a newspaper reporter; it's really tough to imagine a more American job. Clark probably spends the bulk of his days going to school board meetings and sitting in on zoning hearings. For the two hours he spends listening to some Metropolis city commisioner debating a new subway line, he could be out fighting crime, rescuing airplanes, or venting volcanoes. But, he's made the decision to write the article about the subway line over saving some village in South America because Clark understands that, in his ordinary job, he is an essential cog in the great machinery of democracy.
Yeah, there are going to be days where Clark has no choice but to put on the cape and go out to beat up Gorilla Grodd and his zombie monkey army. And, sure, if a plane is falling out of the sky, he's one of about a dozen people in the DC universe who can stop it. But I'd like to think that Clark, as Superman, has seen a lot of the world. He's seen countries where children are starving, other countries wracked by civil war, and plenty of places where the leaders are nothing but thugs who enrich themselves while their citizens suffer.
Clark understands that the real problems of the world aren't going to be solved by men in tights swooping in and beating up people. Instead, civilization works best when governments are freely elected and operate transparently, under the watchful eye of a free press.
Clark Kent punches the clock and works eight hours a day reporting on the boring mechanics of government because he believes that in doing so, he's building a better world. I imagine he's frustrated daily with a lot of the idiotic things that government does. He fights his most important fights by sitting down and typing, so that the people can know what is being done in their name.
Superman is a demi-god who can't be bothered with politics. He owes his loyalty to no country, and pays no taxes. Clark Kent is a citizen of the United States of America. He votes. And I bet he pays every last penny of his taxes, happy to chip in his small part toward the purchase of civilization.