Students on our e-course site very much enjoy S:P’s PM plugin and make good use of it. One thing that concerns me a bit is that PMs are triggered by usernames, which are revealed in the browser bar. Our site is member’s only so the concern isn’t that dramatic, but still.
Now, we also use a plugin called User Profiles Made Easy (http://codecanyon.net/item/user-profiles-made-easy-wordpress-plugin/4109874) to provide students with a front-end profile editing option (which works in tandem with S:P profiles). This plugin pulls users by ID rather than username, e.g. http://xxxxx.com/profile/?viewuser=4615, which is WP’s user ID. This got me wondering if the PM plugin could perhaps do something similar? Just curious.