This is just like the iPhone 3G > iPhone 3GS transition. Doing a new enclosure every year would be pretty taxing, so they take a year off. Incremental hardware improvements meanwhile. The funny bit is they barely needed to do this much — iPhone 4 was still selling steadily.
I think I would prefer an iPhone if it were on T-mobile or if not that, at least the option to buy an unlocked iPhone at a reasonable price that worked at reasonable speed on T-mobile. Not so with today's announcements. An iPhone 3G can be purchased unlocked for $375, or an iPhone 4 for $549 but both are very slow on T-mobile at EDGE data speeds.
An exciting announcement for me personally would not have been a new enclosure, but rather the ability to run on T-mobile at fast speeds, all at a reasonable price. Wishful thinking, I guess.