I remember Davies saying once that he didn't like the idea of doing a multi-Doctor episode because then it becomes more about the actors than the story. Granted, this whole finale is all about the actors guest starring but I can't see him doing his last regular episode with a Doctor that he's never written. And I think bringing in Eccleston (the only Doctor Davies has written) might seem like an insult to Tennant so I can't see them doing that either.
Of course this is all speculation. Davies also said he would never bring the Master back during his run, and then that he enjoys lying to throw off fans.

Bow ties are coool.