The get out for Tennant (and Piper) is now firmly inbuilt after Journey's End I reckon. I won't be at all surprised to see the 10th doc returning, but hopefully they won't do a multi Doctor episode/special for several years. I wouldn't like 10 to turn up until 11 has had at least a season and a half/two seasons to get his feet under the table.
Yep. It would be
horribly irresponsible of the staff to do a multi-Doctor installment for quite some time. David Tennant is leaving the show as perhaps the most popular actor to ever play the role, having been the veritable
face of the show for more than three years, and establishing himself as one of the most famous actors in all of Britain. For young people, for many new fans of the show, for a lot of Americans -- David Tennant
is the Doctor.
To bring him back merely a year, or even two years, after his departure would be terribly unfair to whomever is tapped as his replacement. That actor is already going to have enough trouble in his first season or two with fans and critics shrieking, "He's not Tennant." The last thing the show needs to do is bring Tennant back as his memory is just starting to fade from viewers' minds, reminding everyone that no matter how good this new guy is, he's not their favorite.