The plus side to Amazon's attempts to differentiate ebook hardware and content is being able to read content on more than one device. However, when one arm of the company is pushing customers to buy more Kindle hardware, and the other has cautious publishers insisting on layers of DRM, confusion is bound to happen. According to Gear Diary's Dan Cohen, that confusion is already upon us: he's spent his weekend and several long calls with Amazon customer care trying to figure out how many times he can download ebooks he's purchased, and being told different things by different employees.
The problem arose when Dan attempted to download ebooks he'd already purchased through Amazon onto his iPod touch, which had just been upgraded to iPhone OS 3.0. While some of the titles would cooperate, others refused; customer services first told him that each book has a limit to the number of times it can be downloaded, and that this limit was both undisclosed (sometimes even to Amazon customer care themselves) and arbitrarily set by publishers.
In trying to corroborate this, and find out how to track the download limits, Dan was then told several different versions of this, until reaching what's believed to be the bottom line. Apparently, books can be downloaded an unlimited number of times, though only active on a limited number of devices (e.g. Kindle, Kindle 2, Kindle DX or the Kindle Reader app on an iPhone or iPod touch). That number is usually five or six, but the publisher can apparently change it to as little as one. Swapping an older device for a new one doesn't automatically release the license, so you'll need to contact customer services to have them do it for you.
Complicated enough, but at least one Amazon ebook publisher has spoken up claiming that nowhere in their distribution agreement or control panel is there the ability to change download settings in the way the retailer describes.