Catherine Tate seems to think so.
The comedian, speaking on Radio 2's Jonathan Ross programme, said: "I think it's maybe David's last series."
The BBC needs to do everything in its power to keep him in series 5 in 2010. Hopefully, despite their cost cutting exercise they're wise to this. The show managed to keep going in 1984 after Peter Davison had only been there for two seasons, indeed Tennant's predecessor Christopher Eccleston notoriously managed only one. But the relaunch is still fresh in the public's mind, especially in an age where the media is so fragmented (alongside viewers' attention), and to me only three seasons plus a year of quarterly specials would be far too short for a man labelled frequently as the best Doctor ever. This blogger's fingers are crossed - at least one full season with Steven Moffat at the helm (as is strongly rumoured) and then give someone else a crack as the Eleventh Doctor.