There is an interview with David Tennant in the latest issue of Doctor Who Monthly and quoted on the BBC Doctor Who homepage.On this he says,
"I think I know how many series I'm going to do,and Russell,Julie and I have talked about it,and will continue to talk about it,but I think it would be stupid of me to say more than that right now - anything can happen,and everything can change..."
I know that's not very enlightening,but at least it's his words.I hope he stays a while longer.