A new interview with Russel T Davies is online, in which he speculates at a possible Torchwood/Doctor Who crossover. He says he is happy for members of Torchwood to join Doctor Who, if the right story appears. Here is what he has to say on the matter:
I think the lines are becoming blurred... now that there's a pre-watershed Torchwood repeat," showrunner Davies told Doctor Who Magazine. "We're building a strong, coherent, interconnected universe... but the answer, as ever, is story. If ever I think of a good story for Doctor Who and Torchwood to meet then I'll do it.You can read the whole interview here!