They don't have much of a choice, when the iPhone goes to Verizon, TMobile is pretty well screwed.
They aren't the cheapest option any longer with Virgin's plans, they don't have much for compelling products that aren't on other carriers, the prepaid business is labor intensive & has pretty regular churn, ATT has a better and faster network, everyone's calling them out over the "4G" claims, and DT isn't exactly putting the resources behind TMobile to make them a compelling carrier, the only reason I stayed is my grandfathered plan and hacked data for $9.
I put my kids on Virgin, and got an EVO.
I just canceled my last line of service with them a minute ago, I felt like a traitor, their customer service was/is awesome, and bent over backwards more times than I can count to make me happy