I'm also very interested in this. It seems like there are FTP-like tools for the backend, but I haven't seen one that restricts a user's access to only a certain folder. I would like to give a user/client access to their own folder where they can upload files, while another client has access to their own folder, and so on. If anyone knows a good way to accomplish this, I'd appreciate hearing it. It's easy to do when setting up an FTP user via cpanel, etc., but I don't know how to bridge that into a web-based, wordpress-based interface.
So far I've found a couple of the backend FTP tools I mentioned, which don't seem to have a way to restrict location access per user, and now I'm trying out cforms. But while cforms will help get files uploaded, it doesn't provide the user directory access which would be ideal. And it requires going elsewhere to add subfolders, and creating a whole new form for each client if I want the subfolders to be used, and so on. I'd really prefer to give site clients/users a way to see and use their own (private) subfolder for uploading files.