Its starting to look like The Next Doctor, who we meet on Christmas day, isn't an conman, Russell T Davies has even gone on record as to say such.
So what if The Next Doctor is a future incarnation of the Doctor? Not necessarily Doctor number 11, but a future one (and we all know he will beat the thirteenth regeneration, the Master did!), and this Doctor has somehow lost his TARDIS along with his memory!
So the story results in Doctor Ten (David Tennant) giving his TARDISto his future-self? But perhaps getting a lift somewhere first? We have been promised a surprise ending after all!
Remember this from a few days ago... He won't just be traveling without a companion - he'll be without the TARDIS too.
So just what is his TARDIS? A shed with a coat of blue paint? A horse and carriage painted blue? OK, I admit I'm being silly, but the more observant among us will remember seeing the BBC mock-up of The Next Doctor DVD Cover, do you remember what was at the top right? It was a hot air balloon.