This is a rumor that cropped up earlier this week, and it goes back to an article in the Daily Mirror.
The Daily Mirror reported that this year’s Children in Need telethon will see a 15-minute Doctor Who segment in which all eleven incarnations of the Doctor appear. This will also serve as Matt Smith’s first on-screen appearance as the Doctor.
According to the report, the Tenth Doctor, played by David Tennant, will be searching for a device of some sort, and all the previous incarnations will be drawn into the search.
The first three Doctors, the actors for which have passed away, will be included via footage from their original episodes.
There has been no official confirmation of this from the BBC, so this one is staying firmly in the rumors department.
Doctor Who has had two previous Chlidren in Need segments. The first introduced David Tennant as the Doctor, acting as a bridge between The Parting of the Ways and The Christmas Invasion. The second segment, airing in 2007, featured both David Tennant and Peter Davison. Davison played the fifth incarnation of the Doctor in the classic series.