Here is a cool concept for a user on the go: an inflatable mouse. A small flexible pcb board which houses all the elctro-goodies, a slim-profile USB connector/cable, and an inflatable plastic body.
When you want to store it, just pull the plug, deflate it, and stash it between the keyboard and monitor on your laptop. Then you never have to wonder where the hell your mouse is
The big question I have is: how thin can you make the PCB and USB? If it isn't thin enough there are going to be a lot of damaged LCD panels!