I love Stephen Graham (Combo in This Is England etc,) but in regards to his Amercian "accent" in the recent Public Enemies... Ich don't think so
Meh. It didn't bother me as a 30's Chicago gangster accent. <shrugs> But what do I know? I would have called him Stephen Graham from Snatch.

On Fringe:
Anna Torv is Australian
John Noble is Australian
Joshua Jackson is Canadian
Except Canadians don't have an accent (unless you happen to live on the east coast), so typically they don't have to change the way they speak.
I know a girl from Manitoba that jas a Hell of an accent.
I'll add Kevin McKidd to the list, he's been on Journeyman and Grey's Anatomy.
Another Rome castmember on an American show (filmed in Canada) is Polly Walker on Caprica - though I don't think she faked an accent in the pilot. Also in Rome is Paula Malcomson, who was amazing in Deadwood.