I've been interested in this for other purposes, but someone else is specifically looking for this to use with cpanel. Is there a way to use a form with the user's login name and password to login to cpanel using php, cgi, or something else without having http://username:password@somedomain/cpanel pop up in the browser window? We don't want that part showing up for security reasons (the browser storing it in history). While they could just go to the url and get the popup, it'd be nicer to be able to pop a sign-in form onto the website for customers to use. Gives it a friendlier feeling to it I think.

Have you seen this been done on any site?
I can't think of a way to do this myself.... :(

Not that I know of. I would have begged the site owner for info on it had I seen it. :) Probably can't be done, but it doesn't hurt to ask.