Correct me if I'm wrong but from what I can recall is..
You can only request 1 "sim-subsidy unlock" every 6 months. Your post paid account must be active for at least 30 days. Prepaid accounts must be active for 60 days with at least $10 balance in the account. If you purchased a phone at full price, you can fax in your proof of purchase to the "Blackberry & PDA support department" for a response within 24-72 hours.
I work for T-Mobile, I'll check the specifics the next time I'm at work.