The official word on the Sci-Fi Channel picking up the second season of the revived Doctor Who says that the September launch will in fact include The Christmas Invasion as part of a two-hour premiere:
Chris Regina, Vice President of Programming, SCI FI Channel, said “Our audience has clearly embraced ‘Doctor Who’ and it has delivered a significant increase in viewers in the time period. We are looking forward to keeping the momentum going with David Tennant as the new Doctor.”
Executive Producer and Lead Writer Russell T Davies says, “We were delighted by the first season’s success in the US, and can promise new thrills, new laughs, new heartbreak, and some terrifying new aliens in Season Two.”
No word on if the two-hour block will include the first episode of the season, New Earth, or will be padded with other material, but I’d suspect the former.