It's great that you can do this, but why stuff this functionality into ssh-keygen? A command called ssh-keygen should only do one thing: generate keys. The command for using an ssh key to sign something should be ssh-sign, especially since one of the ostensible design goals here is to get away from PGP's horrible UI.
ssh-keygen already does a bunch of other stuff, though it all relates to manipulating and using keys. It should probably really be renamed ssh-keytool or something.
