Spoilers!!!!!!!
What I am hearing for Series 4 is Sir Ben Kingsley as Davros, plus Joan Collins as The Rani at some point during the season. For Buffy fans, James Marsters (a.k.a. Spike) does a guest shot in the first episode. It sounds like RTD is pullilng out all the stops for the Series 4 finale, with Rose & family, Martha, Sarah Jane Smith, and Captain Jack all on hand for the final episodes. As I understand it, David Tennant has confirmed for the four specials in '09. The BBC has also already commissioned a Series 5 in '10, but absolutely no casting has been done yet because it is so far out. I gather the reason they commissioned it so far in advance was simply to prevent any rumors of cancellation.
My best guess (and this is *only* a guess) is that David Tennant will probably leave after '09. I base that guess simply on the fact that historically most actors only last three seasons in the role, combined with the fact that Tennant is clearly very much in demand now. I also think it is very likely that Russell T. Davies will leave after the four specials in '09, and that Steven Moffat will take over running the show. He clearly understand what makes the character tick. I only hope that if he really does take over the show, that he will bring back Sally Sparrow as a regular companion. But that's just a fanboy wish. I thought that Blink was hands-down the best episode of Series 3, and probably one of the single best episodes in the entire history of the show.