I just read in Slashdot about how Amazon Uses DMCA To Restrict Ebook Purchases. Basically, someone figured out how to get eBooks from another service onto the Kindle.
As a part of our business, we occasionally have spirited debates about the DMCA, and how it is to be applied properly. Basically, Amazon is trying to use the DMCA to shut out competing eBook formats. The DMCA does not cover interoperability – only copyright infringement – and in my opinion Amazon is clearly in the wrong in this case. No copy protection is being cracked here – a DRMed eBook remains DRMed.
As some of the commenters point out, Amazon’s business model is to make money selling the eBooks, and not the hardware. The Kindle is at best a break-even proposition for Amazon. If people are able to purchase eBooks outside of Amazon, then that destroys their business model to a certain degree.
Another comment articulates the legal position really well:

Leaving aside the issue of users’ rights, as far as I can see Amazon is just plain wrong on the law and lacks legal justification for the takedown notice. What the DMCA prohibits is the distribution of tools for overcoming technical measures for protecting copyrighted materials. The first program generates a MOBI ID from a kindle serial number. The second program rewrites a non-Amazon ebook so that it contains the id that will allow it to work on the Kindle with the given serial number. Neither program modifies or copies the Kindle’s software. Since the ebooks in question are not produced by Amazon, no material whose copyright belongs to Amazon is affected in any way. In other words, this software does not defeat any technical measure of Amazon’s for protecting copyrighted material since Amazon has no copyrighted material at stake here. The DMCA is inapplicable, and the takedown notice invalid. Indeed, it is so clear that this software does nothing to defeat protection of copyrighted material that I would say that the takedown notice was issued in bad faith.
What this software actually does is allow for interoperability, which is explicitly protected by the DMCA.