It seems to me the Bitcoin-qt 'Sign Message' feature is a little harder to use than it should be. Once all of the fields are filled in, it would be more convenient to output a block of text that contains all of the information that should be pasted in to a message. Also a simple parser that expects this format should be put into the Verify Message side of things.
It's worth a note that Armory and MultiBit are working on this feature. It would be nice if they all were compatible.
Here's an example (taken from a PGP Signed message):
-----BEGIN BITCOIN SIGNED MESSAGE----- Address: 1Bitcoinasdfasdfsadfsadfsadf
Because anyone can claim to be me. There's no validation of the user name or email address when someone posts a comment. While I do try to remove imposters, some may slip through. By signing my comments using this technique, anyone can independently verify that I was the author of the message by validating the signature. -----BEGIN BITCOIN SIGNATURE----- Version: Bitcoin-qt (1.0)
iD8DBQFFxqRFCMEe9B/8oqERAqA2AJ91Tx4RziVzY4eR4Ms4MFsKAMqOoQCgg7y6e5AJIRuLUIUikjNWQIW -----END BITCOIN SIGNATURE-----