I have used keyboards that are projected onto a surface with a laser light before and they sucked. I have also used a few pico projectors and while they are cool and work in dark rooms, they aren't ready for the big time just yet.
A new design concept submitted to a Dell design contest by Pauline Carlos has turned up today using both a pico projector and a projected keyboard. The thing looks cool, but with the tech available today it just won't work in most instances.
The concept is constructed from a biodegradable starch-based polymer and lacks a mouse. In the future Dell computers will just read your mind I guess.