Not really. Apple's relationship with the music industry (and, to a large extent, their handling of iPhone apps) is more like Volvo's relationship to the petroleum industry... if the entire petroleum industry had failed, in spectacular fashion, to come up with a workable means of delivering gasoline to consumers, had spent half a decade suing anyone who tried to deliver gasoline to consumers, and then Volvo had stepped in and opened the Volvo Gas Store. Apple started selling music online because the RIAA wouldn't, not because they wanted to compete with music retailers - although it certainly didn't hurt that the music industry morons accidentally gave Apple enough pricing power to made 99-cent tracks the new sales model.
In other words, Apple is in the business of selling hardware; the music is just a commodity to them, and their only purpose in selling it is to drive more sales of Apple hardware. On the internet bandwidth is already a pure commodity, and Apple's music store is no danger of fading into oblivion: If anything the opposite is true and Apple is dominating online music sales, again thanks to the music industry morons who gave Apple an insurmountable lead (and made the even dumber mistake of allowing DRM that locked the music to the Apple hardware, but that's another story).
It's also telling that iPhone apps quickly raced to the bottom of the pricing scale: If a 99-cent app delivers more than a dollar's worth of value to the customer, then the app has effectively added value to the phone, and Apple pockets the difference in increased hardware sales. If AT&T Wireless became a pure-bandwidth provider, the only thing Apple would do is to stop turning away apps AT&T doesn't like - Skype, Google Voice, Slingbox, etc. - and let those apps add value to the phone as well.
The only thing that might endanger Apple's walled-garden approach to selling iPhone apps would be a competitor with a wide-open app store that attracted more developers, led to more interesting apps, and threatened to reduce the value that third-party apps currently deliver to the iPhone. The Nexus One is a signal that Google wants to go there (and, in passing, that Verizon and other carriers will fight tooth and nail to prevent opening their networks), but the most likely outcome here is pressure on Apple to make their app rejection policies more transparent and developer-friendly.
So I don't think there's a scenario where Apple's music and app stores fade into oblivion, even if wireless bandwidth becomes a commodity - again, bandwidth is already a commodity on the wired internet, and both the music store and the iPod Touch are thriving.