It's possible that Smith won't even be there for Tennant's regeneration scene, inasmuch as they filmed Eccleston's scene and then simply added Tennant once they were ready to start filming for the Christmas Invasion. Given that the Doctor may not even have a companion present (and may even be in the TARDIS again), the final escript may simply say "The Doctor regenerates..." and that's it. The producers will lock down the camera angles, call it a day, and then go back to work in July when Smith is ready to go for 2010.
Mark
Mark
