The Doctor Who BBC web site is reporting that Doctor Who Monthly has Russell T Davies explaining that he has the 2008 series in the planning stages now even though it has not yet been commissioned by the BBC (see BBC - Series four plans - Russell T Davies talks about the future). RTD states that he expects that a 2008 series will indeed be commissioned by the BBC and the production team needs to be prepared.
Meanwhile, the tabloid The Sun is reporting that David Tennant has signed for the 2008 series ("Series Four") in a £1 million deal. The tabloid goes on to say that he is not expected to stay in the role after that as "he is being flooded with film offers." It claims that an 'insider' has said, "David absolutely loves the programme but if it comes back for a fifth season bosses will have to find a new Dr Who." See The Sun Online - Tennant to return as Doctor
As always, with any reports in the tabloid papers, please consider the source.