This would be interesting.
I spent a couple hours playing with my dad's Kindle. I searched, found, downloaded, and read the trial chapter of John Scalzi's "Old Man's War". Then, after my dead bought a Philip K. Dick short story collection, read two short stories. I was fairly impressed. After getting over the device's physical awkwardness (after nine months of iPhone use), it was a pleasant experience. The device-store integration is part of what I like so much about my iPhone. And the ability to trivially get free and paid content onto the device meets or exceeds my expectations.
And as I've considered the comments in the "How to Beat Apple" thread, and my own "you've got to be awesome" opinion, I feel that it's not about individual hardware or software features, but the collective experience. Apple gives me a world to play in, software, hardware, content that all work together easily. And I can also put my own stuff in too, easily (be it ripped DVDs or Kindle books).
And then there's my experience with Verizon, seeing what happens when company with no sales-reason to be a good retailer tries to become a retailer: FAIL, as the kids say.
So Amazon: they've got it all. They're a killer retailer and are known for high-quality customer service. They've got fingers in all media and have been running the largest ebook store for a couple years now. And they recognize that they're essentially a giant online, web-services supercomputer with a retail store bolted on; that portends the ability to launch a full-featured "cloud" system to service mobile devices. They've got some experience with cell carriers, having negotiated the Kindle's free wireless access. And they've got experience in actual hardware, with the Kindle.
Amazon could do it, perhaps. But I won't guess if they can really pull it off. It will be interesting to see. :)