How does Amazon know what device you've registered

…and other questions.

Does anyone here know the ins and outs of the new Amazon cloud system?

I share a computer with my parents, and I’m afraid this will cause problems. We’ve all bought MP3s and have them in our own cloud.

I “authorized” the computer today from my account. Does this mean the computer can only download from my cloud?

Also, does anyone know how they recognize the device? As far as I can tell it’s not through any kind of cookie. I found a discussion about google music that guessed they were storing the MAC address. Does anyone know if that’s what Amazon is doing?