Do exactly what I am going to do: I am currently on T-Mobile's Solo 20 SIM-only plan, so once I get N900 with new T-M contract in my hands, I'll port my current number to Vodafone's pay-as-you-go service and once that's done, then port it back to T-M contract that came with N900. I reckon the whole operation shouldn't take longer than few days, maybe a week.