A Better Way
I haven't done this myself, but I talked with a very efficient, knowledgable and helpful Tmobile store manager today and we worked out a way to do this:
1. Get a Tmobile activation kit and sign up for the $30 plan.
2. Go to the Tmobile store and say you want to buy a full price, $580, iPhone, and that at the same time you want to top up your current account by $60, giving them the phone number for your $30 prepaid plan.
By charging you the $60 for the top up, they are allowed to sell you the phone for $580, just as if you had signed up for a $50 or higher postpaid plan. But this let's you get the phone at that price by put the money toward the prepaid plan. Tmobile prevents people from buying devices from their store that they never meant to use on Tmobile's network (and flipping them for a profit on eBay or craigslist) and you, the end user, get the phone for the cheaper price.
The manager also confirmed that by paying full price, you could immediately request on unlock from customer service. (And this fits the new online tmobile unlock policy if you pay full price you do NOT have to wait for a time limit.)