Although Doctor Who (and, by association, Tennant) does have a following in the U.S. Perhaps not a large audience (certainly not, say, Daniel Radcliffe large), but it's better than starting off with absolutely nothing, like a show might if their actor/actress was new and unknown to everyone (not just citizens of a certain country).
I've always wondered why they cast British actors for American parts, though. It seems like an odd thing to do. Mostly I'm thinking of Hugh Laurie in House.
At the very least, I would love to see the pilot just to see if it really is good. That wouldn't happen, but still. You never know. Could be great, could be horrible.
Even in Doctor Who, I've seen some horrible episodes. (Well, horrible in my opinion, anyway.)
"I'm sort of like a child genius without being a child or a genius."~Tim Rice-Oxley