Well, the new DWM magazine appears to have hit subscribers earlier than it should - and hence, some 50th Anniversary casting news has made it's way onto the internet.
DWM confirms that David Tennant, Billie Piper and John Hurt will feature in the Special with "further casting details a closely guarded secret".
Let's hope we get more Doctors - and not just Ten - appearing with Matt in the Special.
Edit: Official BBC Doctor Who twitter account have just posted photos of David Tennant and Billie Piper with the hashtag #DoctorWho50th - best confirmation you'll get I expect!
EDIT 2 - Official confirmation from the BBC: BBC.co.uk