So it looks like Catherine Tate jumped the gun. Can this blogger say whew?
Tennant's refuted her claim, saying he is going to be the Doctor for the three 2009 specials, and that as usual he's not yet signed to season 5 because he hasn't yet been asked. So my suspicion that Tate's claim was part of his usual bargaining strategy was probably right. The BBC will find it hard to avoid signing him to season 5 now, whoever is in charge of it (according to Peter Davison it will be Steven Moffat).