Of course, Tennant using an accent is nothing new. While playing the Doctor, he used a very different accent from his native Scottish one. If you happen to listen to the Big Finish Doctor Who audio adventures, you can hear him using a German accent as well.
This particular accent for the American series was kind of cool in a nerdy sort of way, being somewhat high-pitched and nasal. It makes me even more sorry that this series didn't have the chance to shine in the American markets. Video is after the jump.