According to posters on Outpost Gallifrey, the new Doctor Who Magazine reports that Doctor Who will be going High Definition for the four specials that are seeing out the Tennant/Davies era.
The production team had always reportedly been resistant to the show going HD because of the extra costs and time involved in making the FX HD-compatible. They have also expressed concerns that the TARDIS set might not stand up to HD scrutiny. Which begs the question – is a new TARDIS interior being built? Or will we simply not see it in the specials, and we'll only see a new TARDIS interior with season five? One thing to bear in mind – the winners of the "Go on the set of Doctor Who" compo arranged for last year’s Children In Need were sworn to secrecy about about a few things. Maybe a new TARDIS set was one of them...?