I’ve just finished doing a fresh Gutsy installation with an encrypted root file system. I followed this tutorial. Although everything was strictly followed, in the end all I got after reboot was:
ALERT! /dev/mapper/root does not exist. Dropping to a shell! BusyBox v.1.1.3 (Debian 1:1.1.3-5ubuntu7) Built-in shell (ash) Enter 'help' for a list of built-in commands. (initramfs)
This forum post pointed me into the right direction to solve the problem. Also I had to rebuilt my initramfs and update GRUB’s menu.lst. More information can be found here.
Henry's stuff 