At the moment, this remains something of a rumor, since the only evidence for its existence is IMDb, no word from any actual person involved in the production of this year’s Doctor Who finale has come down the pipe. But yes, it is a spoiler about this fall’s Doctor Who finale, regarding casting.
If you’d like something less spoilery and more likely to be actually true, how about this:
[The Daleks] aren’t going to make an appearance for a while. We thought it was about time to give them a rest… There’s a problem with the Daleks. They are the most famous of the Doctor’s adversaries and the most frequent, which means they are the most reliably defeatable enemies in the universe.
Which was said by Who writer Steven Moffat to BBC News.
If you’d like to head down the probably to be disproven even though it’s totally awesome wormhole, click on, brave traveller.
IMDb is currently listing Matt Smith, David Tennant, and Peter Davison as appearing in the finale episode as the Doctor.
Now, as Blastr readily points out: “you’d have thought that Steven Moffat and company would have waited to bring out the big guns (aka former Doctors) for the 50th anniversary of Doctor Who, which is coming up in 2013, and not for a one-off season finale.”
Add to that IMDb’s mixed record on early casting info, and you’ve got some healthy dubiousness to go along with the exciting information, which is the best kind of nigh unsupported rumor, really.