IF one was to do so then you would have the same rights normally granted to someone who buys a paper book and lends it to a friend!
Hopefully Amazon will allow unlimted 1 person at a time DRM soon. I've sent emails asking about it a few times. Basically I want them to offer the same rights as a paper book, so I can 'send' the book to a friend and they can read it, but only 1 person can have the book open at a time.
That's similar to the way 'steam' works for computer games. I can install most games on unlimited machines but only one machine can have the game open on an account at once.