According to digital spy and an interview with RTD...
"Russell T Davies is happy for Torchwood characters to appear in Doctor Who should he come up with a suitable idea, he has revealed.
"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."
Davies does have one caveat about this potential crossover: "The only limit, I think, would be having The Doctor appear in Torchwood itself. Because the 9 o'clock transmission comes first, that would be putting a children's hero into the wrong environment. But anything else is possible!"
Torchwood regulars Captain Jack Harkness and Toshiko Sato both made their first appearances in Doctor Who, featuring in the stories 'The Empty Child' and 'Aliens Of London' respectively. The Doctor's former companion Martha Jones, played by Freema Agyeman, will also star in several Torchwood episodes during the second series."
I thought I'd post this in a new thread because we've discussed the possibility about the team appearing in Who before and I think it'll be a good idea (as long as the Doctor doesn't appear in TW) and I thought we could discuss it more here and discuss possible plots for why torchwood could appear in Who.