I don't think anyone knows for sure yet. If Moffit wrote it, it'll probably be one of those 4th into 5th doctor dealies where the next doctor is part of the previous doctor's regeneration.
I've also done some digging and I think the fired thing is a rumor. Apparently, Tennant has loyalties to Davies and decided to quit when Davies announced he as leaving. Tennant just kept the decision quiet until a few weeks ago.