Okay, so if respeculating my speculation over Paul McGann returning as the Eighth Doctor wasn't bad enough, along comes this interesting little thing.
The Internet Movie Database (IMDb) currently has a cast listing for Episode 13 of Doctor Who Series 6 that includes two very familiar actors -- Peter Davison and David Tennant, also known as the Fifth and Tenth Doctors, respectively. And yes, they're both listed as playing "The Doctor" in the episode.
Other returning names of note include Alex Kingston as River Song (No surprise there), Ian McNiece as Winston Churchill, and Richard Hope as Malohkeh. The return of Churchill and Malohkeh was previously confirmed in Today show host Meredith Vieira's piece on her Doctor Who filming experience.
Now, I know that IMDb gets their information wrong from time to time, so I'm a tiny bit guarded about this, but you have to admit that bringing two previous Doctors back would be a hell of a finale...or perhaps a cliffhanger for the 2011 Christmas special? I don't suppose Doctors Five, Ten and Eleven could team-up to rescue Eight from being marooned on whatever "rock-like planet" he's possibly stranded on? "The Four Doctors"...Say, that's catchy...
Ah, well. I guess we'll find out more later this year, won't we? Two minutes to Belgium!