House's Hugh Laurie isn't the only Brit who can fake an American accent.  Of course FlashForward's Lloyd Simcoe (Jack Davenport) and Simon Campos (Dominic Monaghan) are British.  But it turns out Olivia Benford (Sonya Walger) and Mark Benford (Joseph Fiennes) are British as well.  Sonya Walger had a British accent on Lost, but her American accent on FlashForward made me think she was faking. 

Update:  Turns out Gabriel McDow (James Callis) is a Brit with a fake American accent as well. #entertainment