For those interested, someone posted an excerpt from a radio play/book/? that David did which showcases his American accent. I don't know how old this is, though, so who knows if his accent is better by now or not. I don't think it's too bad, though there were a couple bits where it slipped. But then again, if you go back and watch the first few episodes of House, you can tell how much better Hugh Laurie has gotten at doing the American accent from then to now.
http://www.youtube.com/watch?v=B834-0f4rdY
Joy