Yeah, if it's impossible for a thief to use the computer then Prey is useless. It's why I have a guest account on my computer: if someone steals it, I want them to be able to get it online so that I have some chance of recovering it.
Thats a problem i have as well, filefault 2 is great, but the end user/thief wont be able to report his location via a prey app.
There is got to be a workaround....
The easiest is to make two installs of OSX on the machine. The first has full admin access and prey running. The second has full disk encryption and locked down.
Default boot into the first partition. If the system is stolen they will reboot and you can track. Because it has full admin they are less likely to reinstall.
It is if you make a habit of leaving your laptop laying around unlocked and enjoy writing blog posts to that effect.
Oh, but for actually using the computer? Yeah, somewhat impractical.
(My worry is not the $1000 I sunk on an easily-replaceable chunk of plastic wrapped around my data. It's my data. So my laptop boots up to "type the encryption key", and if you don't know it, you have to erase the disk and install your own OS, keeping my data safe. I'd rather my private data stay private than to take a picture of someone stealing my laptop. Also, my laptop never gets stolen.)
Create another account and give out the password in the login screen (either check "Show password hint" or go to Security & Privacy -> show a message when the screen is locked).
