UMA is voice dialing over IP. It uses your WiFi connection to deliver the call to T-Mobile where they add it to the terrestrial system.
Do you need a data plan - NO. Both of my kids have T-Mobile BlackBerrys without data plans, and they can use UMA. They have no other data plans.
Unless you get a special plan from T-Mobile, please note that these calls still count against your minutes. The key advantage is voice / text (for them) in areas with weak coverage, but with WiFi (like our basement).