There is currently no method in WebGUI::User.pm for getting a user object when you only know the username. I needed that for a project I was working on, so I wrote one (it's pasted below).
It would be stellar if this or something w/ equivalent functionality ended up in WebGUI 7.4.
Thanks!
Committed for 7.4.
cap10morgan, please contact me with your real name so that I can give you props in the changelog, for submitting a patch.