If you have a guest account you can't encrypt your whole harddisk. And if my laptop gets stolen I don't care about the money but about my data falling into the wrong hands.
Set up a honeypot account 'm[ou]m', with a visible password hint to the tune of 'Dammit m[ou]m, THE PASSWORD IS "Susan"'.
Make sure it has no access to the filesystem outside of its homedir, and you could even set some login items to watch for net access and push a notification.
The honeypot account still needs access to the operating system, and thus to the harddisk password if you use full-disk encryption.
Full disk encryption is more or less default for most Linux distributions and OS X Lion. In addition, it's the only sane solution if you want to securely encrypt your data.
Make sure it has no access to the filesystem outside of its homedir, and you could even set some login items to watch for net access and push a notification.