Originally Posted by NatCh
I'm a bit shaky on how mobi's DRM works, but I gather that the files are locked to specific devices by way of the device ID key. So in order to read a DRMed Mobi file on a device (such as a Cybook) I would have to register that device with the seller and download a file that is keyed to that device.
That's correct. Every Mobi device has a unique ID - a 10 character alphanumeric string called a "PID" (I'm not sure what the "P" stands for!). When you buy a Mobi book, you enter the PID on the web site you're buying from and the web site encrypts the book for your specific device. You're generally allowed to register up to four different PIDs on a site, and the book can then be read on any of those devices.
If the Kindle does use standard MobiPocket encryption (which seems highly likely), then the odds are that it just has an internal PID which is being used for the encryption. If someone finds a method of getting at this internal PID, it could then be entered at any MobiPocket bookstore to generate a book which could be read on the Kindle.
What probably CANNOT be done, however, is to read Kindle books on other MobiPocket devices, because the book encryption is done by the seller (ie Amazon) and there would be no way to enter the PIDs of other Mobi devices on Amazon, unless they specifically decide to open up that capability.