The Nexus S (non Sprint 4G) works on T-Mobile, and beyond supporting Wi-Max, the two devices are identical. Clean Google experience, and same Samsung Galaxy S derived hardware.

As for what the manufacturers are doing under the hood, it seems my experience with the Captivate only exposed part of it. A coworker has been rebuilding his phone from scratch, and discovered his stock ROM was using ReiserFS. Pretty crazy, and gives me a new view on why Google is trying to control things more now.