Someday last October I made the decision to get N900 on contract, however I also wanted to keep my current mobile number that I've had with T-Mobile for several years. The trick was that an
online retailer where I pre-ordered the device didn't offer upgrades on T-Mobile - only new contracts. For some weird and wonderful reason, T-Mobile also isn't able to cancel existing connection, free of fixed-term contract and migrate my number to the new 18-month contract. Anyhow, there is a workaround - get a new T-Mobile contract with new number, port my current number out to different network's prepaid service and once that's done (ie. within 2-3 working days), port it back from there to that new T-Mobile contract I've got with N900. Simples.