Big spoiler that's really, really not gonna last.
That everyone's gonna know about before the show even starts showing next year.
Billie Piper's back for at least three episodes as Rose.
As mentioned in the Mirror and on BBC news.
Donna's around all season, Martha comes back around ep 4, Jack's already mentioned he's back at some point and now Rose for 11-13. Thank god the Tardis is so large...
It's sounding more and more like those rumours that RTD was gonna team up all his characters for the end of season blowout before leaving the show are true. Gonna be hard to top this lot certainly, but with 2009's block only being the three one-shots, any new production crew would be able to organise and get up to speed in a reasonable way.
Apparently, fans discovered this for certain last night when Billie Piper turned up for late night filming. Hee! But what's interesting is that Grahame Harper was directing, and he's doing the Doctor-lite episode this year. Which is being written by RTD this year. Intriguing... With all these companions running around this year, it would make sense to feature one (or more!) in that ep.
UPDATE 2: DigitalSpy apparently had a report of someone seeing Donna and Sarah Jane filming together. My head might well explode if true....
EDIT - : Now, with added speculation and info.