InferPoint are showing the SlimMouse off – it’s about the size of a finger so it certainly qualifies as portable. The traditional left and right buttons have been rearranged to be in front of each other, and a scroll wheel is on the side.
It’s innovative but personally I found it pretty unergonomic to use. I’ve not got big hands but I had to scrunch them up to get any sort of grip on the mouse and I certainly couldn’t use it for any length of time. On the other hand I’m not a girl with small hands and long nails – perhaps it works better in that case