It has been confirmed by the BBC that, after Series 5 has run on the UK during 2010, it will be shown over in America later on in the year. The official statement is:
“BBC AMERICA has announced the acquisition of the new season of Doctor Who starring Matt Smith as the Eleventh Doctor. Shot in HD, the U.S. premiere season of 13 episodes, will air Q2, 2010 following soon after its UK premiere.”
This is good news for everyone really... just shows how popular Doctor Who's becoming!