Interesting. I talked to a T-mobile rep about this a few weeks back and I got a different answer.

My issue was an old phone I had tossing around that I wanted to unlock for use in China. What I was told was to put my SIM in it for 45 days and then call to request an unlock--the unlock has to come from customer service and they'll only do it when the phone has been used for at least the last 45 days.

He knew I was prepaid, he didn't ask about account balances.

The 45 days aren't up yet so I can't confirm that he's right.