Information is being officially released now about David Tennant's swansong, the two-parter that will run around Christmastime. The title is "The End of Time". Part one will be 60 minutes long, and part two will be 75 minutes long.
Cast will include:
David Tennant (the Doctor, duh)
Bernard Cribbins (Wilfred Mott, Donna's grandfather)
Catherine Tate (Donna Noble)
John Simm (the Master)
Timothy Dalton (the Narrator)
June Whitfield (Minnie Hooper)
Source
Rumor claims also that the Master's wife (Lucy Saxon, played by Alexandra Moen) will make an appearance. And some leaked set shots showed Timothy Dalton in costume -- he appears to be dressed as a Time Lord. So James Bond really is a Time Lord, which of course we all knew already.
Wikipedia has information on other actors spotted on set. Looks like we'll have a ridiculous large cast again, if this proves accurate:
* John Barrowman – Captain Jack Harkness[3]
* Claire Bloom – TBA[4]
* Camille Coduri – Jackie Tyler[5]
* Karl Collins – TBA[6]
* Timothy Dalton – The Narrator[7]
* David Harewood – Joshua Naismith[8][9]
* Barry Howard – TBA[6]
* Jessica Hynes – Verity Newman[10]
* Sinead Keenan – TBA[11]
* Jacqueline King – Sylvia Noble[12]
* Tommy Knight – Luke Smith[13]
* Alexandra Moen – Lucy Saxon[14]
* Billie Piper – Rose Tyler[5]
* John Simm – The Master [15][16]
* Elisabeth Sladen – Sarah Jane Smith[13]
* Catherine Tate – Donna Noble[17][18]
* Russell Tovey – TBA[3]
* June Whitfield – Minnie Hooper[19]
Mind you, some may only be appearing as sort of flashbacks. The regeneration scenes from "Logopolis" and "The Caves of Androzani" had the Doctor seeing spectral images of former companions and enemies from his current life, sort of a rapid-recap before regenerating. While in "Logopolis" these were mined from existing footage, new ones were recorded for "Caves of Androzani", and they might be doing something similar here.