The rumors of a special story featuring all (twelve?) Doctors are false. The special will feature Matt Smith, David Tennant, and John Hurt.
However, there's another rumor, and this one might have some legs to it.
The rumor says that the BBC has found a mess of missing Doctor Who episodes from the Hartnell and Troughton eras, and is ready to run them on the 50th Anniversary. (Please, let Mission To The Unknown be one of them, I'm dying to see what the Varga plants actually looked like.)
Now, we don't know if this is true or not, but the people close to the rumor have heard it from more than one source.
We go to DEFCON YELLOW.