Originally Posted by pdurrant
For Mobipocket ebooks with DRM there's some method to calulate the Kindle's Mobipocket PID, and registering that with your eBook store will let you download ebooks that will work on your Kindle.
Note that this now only works with "Overdrive" MOBIs from Books on Board and on library ebooks (FictionWise MOBIs probably no longer work), see A Beginner's Guide to Kindle Content
. In the Guide, Nate recommends buying LIT and converting to MOBI but, as pdurrant says, mobidedrm works just as well on DRM-laden MOBI ebooks as on AZW ebooks and the resulting DRM-free MOBIs are readable "as is" on a Kindle.