Encrypting external drives using FileVault 2 on Mac OS X LionEdit

$ diskutil info
$ diskutil info /Volumes/Suanzes/
$ diskutil cs convert /dev/disk2s2 -stdinpassphrase
$ diskutil cs list
$ diskutil cs changeVolumePassphrase /dev/disk2s3 -stdinpassphrase

A note about OS X Mavericks

This has become considerably easier on Mavericks; just right-click on the volume in the Finder, choose "Encrypt Volume…" and follow the prompts.