Hey guys,
I just wanted to offer that I was able to do this entire process in an actual T-Mobile store today. I had previously visited a T-Mobile store asking about this plan and they had told me they had nothing to do with it, that it was a WalMart only thing and I had to go there. However, when I went to Walmart, they said they couldn't sell me the plan without a Sim Card, and the only way for me to get a prepaid Sim Card from them was to buy a 30 dollar Prepaid phone and take the sim out. I had read about buying a Micro Sim through T-mobile's website, but last I checked they were sold out.
So today I went into my local T-Mo store, I asked about Pre-paid and she said they could do that, then when I asked about the 30 dollar plan she said she had just set one up that morning and could do it for me. Sure enough, they gave me a Sim card for 10 bucks, and then set me up on the plan. I think basically they just give you the Sim card and then go through the process on the website that you would go through, but having them do the whole thing made it much easier and I didn't have to wait for it all to ship etc.
I brought the Sim home, tested it in my gf's phone while I wait for my Nexus 4... grrr.... and it worked great! So if you go into a T-Mo store and they say they can't do it, you can say that you know other stores are and all they have to do is sell you a Micro Sim and set it up for you on the website.