I occasionally work with clients who need to have Macs set up as kiosks. That means that anyone can walk up and use the Mac to watch a video or access information, but they usually can't update anything on the Mac. Kiosks are especially useful when the user interface consists of a finger touching the screen. In my experience, touch screens are generally expensive and require that you send a Mac off to a vendor for installation.
Troll Touch now has user-installable touch screens for the 20" and 24" aluminum iMacs called SlipCOVERs. Priced at $699 (20") and $899 (24"), the SlipCOVERs come with touch screen drivers and calibration software. The video above shows a touch-enabled iMac in action -- I especially liked watching the demo dude playing Bejeweled using his finger instead of a mouse. Pull off the aluminum stand, lay that SlipCOVER iMac down on its back, and you've got a 24" iPod touch!