According to io9 and Digital Spy, not even the actors know. In an obvious and smart move, the BBC were highly secretive over the filming of Tennant's final Doctor Who episodes. The actors weren't even allowed to see the full script. Obviously Tennant and any actor immediately involved in the regeneration scene are aware. It'll be interesting and a bit sad when the episode finally airs in the UK later this year.
How do you think Tennant's Doctor should go out? Would catastrophe could bring on his next regeneration? Have you accepted the fact that Tennant will be gone and Matt Smith will be taking over?