Billie Piper, Catherine Tate and Freema Agyeman will appear in the final Doctor Who episode starring David Tennant as the Doctor. They will reprise their roles as Rose, Donna and Martha, respectively (though how they are gonna bring back Rose and Donna is beyond me). The episode, scheduled for later this year, will also mark the end of Russell T Davis' tenure as head showrunner (what, you mean "The Stolen Earth"/"Journey's End" wasn't a big enough send-off?!). Tennant's Doctor will face off once again against arch-nemesis the Master before regenerating into the Eleventh Doctor, played by Matt Smith.
And this coming April 12th - Easter Sunday - will be the next Doctor Who special. "Planet of the Dead", incidentally, will be the 200th Doctor Who story since the show began in 1963. Mash down here for some pics of the upcoming show... which will doubtless be on the Intertubes for downloading a few hours after it premieres for our Brittish brethren!