At least locally US Cellular is another sorely lamented absentee. I can't remember why T-Mobile is late to the party but US Cellular said they won't adopt an iPhone until it works on faster networks. Funny, AT&T's 3G is still the fastest overall (I don't count spotty "market" cities as networks; I'm looking for the best overall coverage). Locally, US Cell has the BEST coverage; its customers want the iPhone and there's no 4G in the area. I'm not sure how holding out is helping US Cell. It's obviously not helping their customers.
As an aside, that day Steve announced the iPhone five years ago, Apple's plan was to get a mere 1% of the smartphone market. I've often wondered if this was shortsighted or an earnest goal that was obscenely under-forecasting the might of their product.