You may not have noticed it, but it prompted you for your password as part of the installation wizard. I assume that after that, it saved your password where the boot program could find it. Other full-disk encryption programs have similar capabilities, where they'll automatically unlock an encrypted volume if so configured. This is totally insecure, of course, but in the context of the Mavericks upgrade, it's temporary so I guess that's OK.
The interesting question is this: How well did Apple sanitize whatever bit of disk or flash memory ended up storing the key through the two reboots Mavericks needed to do to complete the upgrade?
Oh, I did notice. The same prompt you get when installing any software. However, my user/password and the disk decrypt password are different passwords, no?
The interesting question is this: How well did Apple sanitize whatever bit of disk or flash memory ended up storing the key through the two reboots Mavericks needed to do to complete the upgrade?