In October 2011, T-Mobile said "for $30 per month and no annual contract, customers can enjoy unlimited Web, with the first 5 GB at up to 4G speeds; unlimited text; and 100 minutes of talk, with 10 cents per minute after the first 100 minutes" at http://newsroom.t-mobile.com/articles/walmart-and-t-mobile-introduce-exclusive-no-annual-contract-4g-offering
To sign up for this plan without visting Walmart, purchase a micro-SIM from http://prepaid-phones.t-mobile.com/prepaid-phone/T-Mobile-Micro-SIM-Card-Activation-Kit or mini-SIM from http://prepaid-phones.t-mobile.com/prepaid-phone/T-Mobile-Prepaid-SIM-Activation-Kit and phone of your choice, such as the Nexus 4 ( http://www.google.com/nexus/4 https://play.google.com/store/devices/details?id=nexus_4_16gb ) or the http://en.wikipedia.org/wiki/T-Mobile_myTouch_4G and then activate the SIM at http://prepaid-phones.t-mobile.com/prepaid-activate with the following three pieces of information:
Activation code (e.g. 8273 381 6274) from the booklet that came with the SIM card
SIM card serial number (19 digits) from the plastic card that holds the SIM card.
Phone serial number (IMEI) probably after popping off the back cover phone: http://www.ifixit.com/Teardown/Nexus+4+Teardown/11781/
You'll probably want to use whatever phone number they give you for a little while to make sure you have coverage in the places you care about. When you're ready to transfer your old number over, you'll want to call T-Mobile's Number Transfer Center at 877-789-3106. They make ask you for your PIN for your old carrier (i.e. Virgin Mobile) and may request that you contact your old carrier to give them a heads up that the transfer is coming. In my case, they scheduled the transfer for three days after I called at 11 a.m. Pacific Time.