The most popular answer seems to be a Doctor Who movie, but seeing as the script doesn't exist yet, releasing it for summer 2010 would be a rush-job :), so don't hold your breath!
So what are the possibilities for Project #3? It has to be something that is completed this year, while Tennant is still the Doctor remember.
IMHO it has to be something small, it can't be another Sarah Jane Adventures, so Torchwood maybe? Children of Earth is already finished and a June 15th transmission looks likely, and it appears they are making no more Torchwood until some time next year at the earliest, so scratch that.
An audio drama for Radio perhaps? Its certainly a possibility!
But how about Children in Need 2009? He has done a 2005 and 2007 special for them, so 2009 is a possibility!