Unlocking T-Mobile Prepaid Phones
You may request an unlock code for your handset after 90 days. You must have made a deposit of minutes (top up card or a PIN) within the last 30 days. T-Mobile will attempt to obtain the unlock code for you. It may take as much as 48 hours for T-Mobile to obtain the unlock code for you. Be advised that T-Mobile will attempt to get the unlock code for you, but it is possible that they may not be able to get an unlock code for your particular handset. In that case the only alternative you may have for unlocking your handset is to take it to a business or individual who specializes in unlocking handsets.
Added 6/13/2010: The current T-Moboile unlock statement (see http://support.t-mobile.com/doc/tm51885.xml) for prepaid says:
For customers with a T-Mobile Prepaid Plan, T-Mobile will provide the SIM Unlock Code upon request to eligible customers, provided the requesting customer has a minimum of 60 days of active service with T-Mobile and either a Prepaid Plan account balance of at least $10.00 or a prior refill within the last 30 days.
I chatted with an operator today who created an unlock request for me without having deposited minutes for 6 months.
Added 01/08/2012: Per customer service, the SIM must be in the handset to be unlocked: "T-Mobile policies and procedures state that the device must be in use on the mobile number requesting the unlock code. If this is a new device please allow 3-4 business days prior to submitting a new request to allow your usage to update in our system."