How do I launch an app at start up but as a specific user? I want to launch an app at login but have it running as a specific user. Here's the use case:

I turn on my Mac and login in as Ben. In the background iTunes is launched for Bill. Ben can launch his own instance of iTunes and access Bills songs via iTunes sharing. Ben doesn't care have access to Bills instance of iTunes. All other iTunes on the local network can also access Bills shared iTunes library.

Please don't suggest iTunes specific solutions (e.g. Home Sharing) as I also want to run other apps in a similar fashion.

Is this possible?
If you just have two users, you can auto login as Bill, have Bill's account auto-launch multiple apps (see launchd, or via the GUI under User Accounts), and after a timed delay switch users (again, launchd or an Automater App + "sleep 30;" + this), returning you to the login screen. You now manually log in as Ben.

If you want multiple apps running under multiple user accounts in the background, it gets a bit more complicated. Please clarify if this is the case.