From here: http://hotspot.t-mobile.com/freeweekends/Quote:
Visit any of our 6,500 T-Mobile HotSpot locations on the weekend and launch your Web browser.
Visit http://freeweekends.hotspot.t-mobile.com and enter your T-Mobile phone number.
Within moments you'll receive a free text message on your T-Mobile Phone with a pass code. Enter the pass code and you'll be surfing for free!