Traditionally (and to this day) mobile connection providers don’t want to become ‘mere’ data pipes, as it becomes difficult to differentiate yourself from competitors. To this end the mobile companies have been furiously ‘value-adding’ for the past 15 years (given a certain definition of value). In the UK at least this has lead to operator branded phones, an operator defined catalogue of phones available to buy, operator ‘enhanced’ phone firmware, operator apps, highstreet operator-run shops and plenty more failed projects including music stores, walled content gardens, etc etc.
At least Verizon is well-poised to be the one carrier that truly benefits from the move being "mere pipes", simply because they have long been the US carrier with the "best" coverage and reliability.
I’m not USian so I have no direct experience, but the reviews of the Verizon iPhone4 are showing about half the upload and two thirds the download speeds that users are getting on AT&T. Is AT&T’s coverage really bad enough to warrant that drop in speed?
Although AT&T advertizes itself as reaching 99% of the US population, the signal does not seem to penetrate buildings, basements etc. as well as Verizon's does.
Coming to congestion,the iPhone led to heavy 2G and 3G usage by subscribers and AT&T's network in highly dense areas really struggles badly with the data congestion.
