I stumbled across this while researching other things and it looks like it would be an ideal solution for clients to submit files. http://www.dbinbox.com
You need a dropbox account (which can be free), and a dbinbox account (which also can be free, but even the fancy version is cheap).
But when you get it all hooked up you get a simple URL you can provide to anyone and they simply drag a file onto the webpage and it appears in a folder on your local computer and you get an email notification.
The client doesn't have to register anywhere, and no one but you can see any files that get sent.
You'd probably have to buy the $30/yr plan to be able to handle full resolution files. But that also lets you embed the uploader on your own website (or presumably within emails which would be very slick).