According to a translated writeup of the Kindle hack here, Amazon engineers went to considerable lengths to prevent their DRM from being tampered with. The Kindle for PC uses a separate session key to encrypt and decrypt each book "and they seem to have done a reasonable job on the obfuscation," the author says.
The crack comes courtesy of a piece of software titled unswindle, and it's available here. Once installed, proprietary Amazon ebooks can be converted into the open Mobi format.
Personally, I think that DRM is only a headache. Amazon is probably making enough money on selling e-books, so why not allow them to be portable anywhere? Making books more available would make a bigger impact on shrinking the size of market of illegal ebooks floating around, instead of adding new protections here.