If you can believe The Sun, Billie Piper’s leaving Doctor Who at the end of the season, as foreshadowed in The Satan Pit. Her replacement has already been picked.
The article’s got quotes from Billie and RTD, so it’s probably true, but you never know with The Sun. There’s nothing on the Beeb’s Doctor Who site or its news site yet.
UPDATE: There’s a press release now. Thanks Scott!
UPDATE 2: Now the Beeb’s news site has a story about it. The door, apparently, is open for Piper’s return. Who reckons this is all so she’ll be a surprise regular next season?