Billie Piper appeared on on Graham Norton's chat show on BBC 1 tonight, and was asked if she was returning to Doctor Who for the show's 50th anniversary. She replied with an unambiguous "no," then clarified that she hadn't been asked.
Neither has co-guest Frank Skinner, who has asked his agent to enquire if he could appear in the show in any form. They received a letter back acknowledging Skinner's interest... but that was all.
You can see the clip here.
UK viewers can see the whole show on the iPlayer.