That's what the Moldable Mouse from Lite-On Technology is all about. It is made of modeling clay, covered with fabric. Knead it into any shape you like! The only rigid part is the base, where the batteries and circuit board are. Wireless technology makes it all possible, as the click buttons and scroll pad are powered by RFID and can be stuck on anywhere. Once molded, the shape is self-retaining. That is, as long as you want it to be. If you decide your shape isn't perfect after all, or when your hand gets tired and you want a change, you can mold it again.
The Moldable Mouse won a red dot award for design concept in 2007. It's not yet available at your local computer shop.