There is a way to do it - It was in Macworld a few issues back. It's a workaround though, involving setting up a new account, copying files, changing names, etc, etc.... Sorry, can't remember more, but it IS possible.
I would think the way to do it is in Netinfo Manager. Navigate to users and then the user account you want. Click the lock to unlock and enter an adminstrator login.
Change name, which is the shortname, to what you want.
But be very careful, you can easily screw up the account if you make too many changes here. You could also change the home directory and copy everthing over, but you may or may not need to change permissions after the move.
I haven't tried this, but it seems to me this would be the way it could work.