There are four main GSM frequencies used worldwide. The Americas use 850/1900 while the rest of the world uses 900/1800. There are four main UMTS frequencies used worldwide. The Americas generally use 850/1900 while the rest of the world generally uses 900/2100. T-Mobile and few other carriers are weirdos and use 1700/2100 for UMTS. The iPhone is quad-band GSM and covers all frequencies world-wide. The iPhone, iPhone 3G and iPhone 3GS were only tri-band UMTS, so they covered 850/1900/2100. This resulted in relatively poor coverage overseas. The iPhone 4 has a pentaband baseband, but 1700 is disabled.
Then I must have been living in an odd bubble for the past ten years since I haven't have any problems with AT&T in the Bay Area for the past ten years.
The original iPhone was sold under a similar model. You paid full price for the phone and had access to a discounted plan, although you still had a two-year contract. While sales were good, the iPhone 3G, which followed the traditional subsidy model was far more successful.
Google also tried the "full price phone for non-contract use" model for the Nexus One, but it didn't work so well, either (the lack of a proper distribution channel and abysmal product support were definitely contributing factors). That said, the Nexus S doesn't seem to be doing too well, either.
I think to Americans, the idea of paying $600 for a phone is absolutely unfathomable, even if it were possible to pay less on the monthly bill. I can't imagine the uptake on T-Mobile's Even More Plus plans to be very large.