Up to Three different iPhones can work together to make a cool virtual visual dj system. Preview the output on one, mix on the other two. Zoom and pan, spin and blend using the touch screens.
Bring together 4 different touchscreen computers (or iPhones, itouches, etc) and they can all connect into one larger canvas. Like the way you can use a bunch of tv's together to make a big screen.
A web 2.0 service that lets you know when a "colleague" is in the same wifi cafe. Aware of itself and others like it, the app gives out your virtual biz card and tells a bit about you and what you do while doing out-of-office work. If you want some comraderie and perhaps some collaboration, you can put up your flag and see who else is up for it. Hell yeah, web 2.0!!!