Yes, I've heard the argument many times - "Superman's an alien, so who cares about the nationality of the actor playing him?". Still seems like he should be played by an American, though...maybe it's just because of me being used to him being played by Americans in animation and that
"Truth, Justice, and the American Way" catchphrase being stuck in my head. Like I said, though, doesn't really matter since Brits do convincing American accents all the time these days, and it didn't hurt Christian Bale's performance.