I’ve been doing some trawling of the internet and according to RTD himself in this link http://www.bbc.co.uk/doctorwho/s4/news/latest/080917_news_01. David Tennant will meet a person who claims to be the Doctor in the Christmas Special. This character will be played by David Morrissey.
RTD also tips Russell Tovey as a possible successor to DT. I suppose whether this will come to pass wil depend very much on Steven Moffat, when he takes over the helm.
Wikipedia gives this information on the Next Doctor
Sorry about the dodgy hyperlinks they don’t seem to work in the same way as they used to.
I don’t know much about David Morrissey, but from the stills of the Christmas Special he might be able to carry the character and replace DT when the time comes. Assuming of course that he does replace DT.