Where I live it is offered through my cable system, although it isn't a part of the basic package. I think the same goes for the various Satellite services (DirecTV, etc). Other than that, I think that PBS occasionally carries some BBC content, but not necessarily on a regular schedule.
On the radio I think the only regular carrier of BBC programming is National Public Radio (NPR). http://www.npr.org/
Hope this helps!
SkyTeam: The alliance for third rate airlines finally getting their act together!