This is a momentous episode for all Doctor Who fans. Never before have all the Doctors been in the same room, let alone in the same show. It's been a logistical nightmare getting all the actors together and available for shooting on the same days. But the script has already been started and it's classic Doctor Who – really witty and very sharp. Viewers will see the Time Lords regenerating and emerging one by one from the Tardis, each with their own quirky opening line. David Tennant is the central character in the episode. He's trying to hunt down some special time travel apparatus. What he's actually lost has not yet been decided, but the idea is for all the other Doctors to club together to help David find it. The actors who are no longer with us, William, Jon and Patrick, will make special cameo appearances, with footage from original shows being flashed up on screen. It's a really exciting project – and all for a good cause.Source: io9
UPDATE: Looks as if the BBC are denying this story A spokesman told the newspaper: "Nothing has been finalised yet, although there is discussion of a Children in Need Doctor Who special. It is too early to say what."
Source: Digital Spy