Chaos Manor Reviews posted about a wireless laptop mouse called the MoGo mouse. The mouse folds up and slides into the PC Card or Express Card slot (two versions, one for each type of slot) when not in use. While stored in the slot, it also automatically charges itself.
I haven’t tried it so I can’t say how well a relatively flat and diminutive mouse works. In addition, if your laptop doesn’t have Bluetooth built in you will need to get an adapter (from them or, I assume, anywhere that sells Bluetooth adapters). But I have to say it is certainly a cool looking idea.