Full Disk Encryption Without Erasing Data in Lion

While reading John Siracusa’s excellent exhaustive (and exhausting) Lion review, I noticed that you can indeed encrypt an existing volume without erasing the data already there. Currently, only the boot drive can be encypted or decrypted in place using the FileVault security prefpane. But John points out that the diskutil command comes loaded with all the muscle you need to do this yourself in the Terminal.