I run Linux on my netbook with an encrypted home directory (decrypted when I log in). One idea I had (partly from Cory Doctorow's Little Brother) was to have a password that I could enter which would login to a fake user account while performing a command (e.g trashing the contents of the disk drive or changing the encryption passwords to something random and very long).
Any ideas how to do this? (Answers involving obscure kernel modules etc are welcomed, though as always a nice command line utility might be a bit nicer! I especially like to have the same username but not the same password: user bob signs in with password ABC and gets logged in, but user bob signs in with password 123 and gets his stuff deleted.)