Shadowy Man
Contributor
When I watch Hugh Laurie in House, I've heard he does a really good job at an American accent, but I can't tell you myself how true that is. It could be the same as the horrific attempts at Irish accents I've heard from non-Irish actors myself.
Though not quite as varied as places in Europe, America does have its variety of accents, and, with people moving around a lot, it's possible that aspects of one's speech has residual geographic-specific accent. So, I would say that when a non-American does an "American accent" as long as it isn't obviously non-American, it's pretty good. Unless they're trying specifically to do one of the geographic-specific accents. I haven't watched enough non-American television to see non-American's trying an American accent, but there are definitely some very good examples of well-done accents, like my aforementioned example of Olivia D'Abo.