I think UMA for those phones are controlled by hardware in the phone. It would be cool if the new iPhone 4 with its faster processor could use software to emulate the UMA process.
If T-Mobile USA in the future is able to get Apple to design an iPhone for their network, then and I think only then will the iPhone have a chance of getting UMA support. The other carriers in the USA see UMA as a threat to their revenue stream. They rather sell additional services like AT&Ts MICROCELL or Sprints AIRAVE.
The closest we will every get to UMA on an iPhone I fear will be if someone can hack a microcell unit.