I think Tennant is sticking around. He, along with Davies, are producing a quartet of feature-length Doctor Who specials to air over the British bank holidays in 2009. While Davies is officially leaving, there hasn't been word on Tennant leaving. There are rumors, and while it's possible the Doctor can regenerate during these specials, there hasn't been substantial evidence that Tennant won't stick around for the 2010 series, which will be a full 12 to 14 episode arc.