During the tenth Doctor's adventures he was able to save the running torch from falling on the ground and then ran the torch up to the main torch and lit it to start the 2012 summer olympics in London for this summer's olympic games.
The Doctor and his then companion visited this time and area to watch the start of the even, but wherever the Doctor goes trouble just seems to appear. During this time the Doctor finds out that an immature Isolus (A tiny alien that travels through space) has crashed to earth. The Isolus then takes control of a young girl and creates havoc. The Doctor figures out that the Isolus crashed because it lost power to its pod and figures out that it needs heat and emotional strength to power it and throws it into the runners torch to start giving it power. The alien leaves the young girl's body but the pod still needs more power before it can take off. The runner who is headed to the main torch stumbles and so the Doctor grabs the torch and completes the run. This gives the pod enough power to leave earth and rejoin it family.
Enter the eleventh Doctor who during his transformation into the eleventh Doctor is also causing the TARDIS (Time And Relative Dimension In Space) which is his Space/Time travel machine to also have a series of explosions. I believe that because of these explosions which to what you could consider the engine of the TARDIS that it would also cause the TARDIS to possibly jump from time frame to time frame (in lets say half second clips, thus meaning that nobody would even see or hear the TARDIS doing this in order to understand this you have to know a lot more about the Doctor then we currently know right now. To include the exact moment when the Isolus pod was also crashing to Earth. This would mean that the tenth doctor would never of had to pick up the torch as the torch runner would of probably never stumbled then for whatever reason. The Isolus pod could of easily collided with the TARDIS thus re-powering, because let's all admit that the TARDIS is a lot more powerful then even the Olympic Torch, thus sending the Isolus pod immediately back to its family.
I'm no Russell T Davis or for that matter Matthew Graham but this is just my opinion as to why we won't see the tenth Doctor lighting the Olympic Torch to start the 2012 Summer Olympics. Now of course it would be really interesting to see it happen, but seeing how the Olympic committee most likely take things seriously and see Doctor Who as just a silly show in their eyes I highly doubt that we will see David Tennant lighting the torch. Especially since he is Scottish and they would probably want a proper member of the UK lighting the torch and not just a member of Great Britain