notes.husk.org. scribblings by Paul Mison.

iamdanw:

In summary, kindle books can only be resynced to a kindle or iPhone a limited number of times. The number of times varies book to book. The number of times is not publicly listed. Once you exceed the number you have to buy the kindle ebook again.

From reading the follow-up post, it turns out that the limit is actually on the number of devices; the number of downloads is unlimited per device.

There’s a parallel here: when iTunes music was DRMed, you could authorise five computers to play the tracks. (iPods and iPhones didn’t count, because it’s harder to copy the music to them.) There are obvious differences. Apple made it clear what this limit was (five), when you added a computer (either via the iTunes menus or putting a password into a dialogue box), and it allows you to deauthorise machines (even, once a year, doing a full reset and removing all authorisation).

As the follow-up concludes,

You are able to redownload your books an unlimited number of times to any specific device.
Any one time the books can be on a finite number of devices. In most cases that means you can have  the same book on six different devices.
Unfortunately the publishers decide how many licenses, that is devices, a book can be on at any one time. While most of the time that will be five or six different devices there will be times when it’s only one device.
At the present time there is no way to know how many devices can be licensed prior to buying the book.
According to the customer rep, there is a project to try to get that information available to the customer but it’s not yet available.

The last part is key. If Amazon insist on DRM, then allowing people to register (and de-register) devices would, while not solving its problems, at least give users the tools to avoid being bitten quite so badly by them.