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The way that Mobipocket DRM works is that the device itself has an identifier, called a "PID" - an alphanumeric string. When you buy the book, the bookstore's DRM server encodes the file using the PID as an encryption key, and the device itself decrypts it using its own PID as the decryption key. A Mobi book can coded such that it will work with up to 4 different PIDs (although Amazon only ever encode a book for a single PID).
Obviously this means that the bookstore has to know the device PID before you can download the book. In the case of Amazon, you select which of your registered Kindles to sent the book to, and Amazon know the device PID from their own database. In the case of an independent Mobi bookstore, there's always a form allowing you to tell the bookstore what the PID of your device is.
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