Microsoft Arc Touch Mouse (Black)

Pros: Flat when off
tiny transmitter
good sensor
elegant looks
AAA batteries

Cons: a little sticky
have to get used to it
why black? Hard to find in a briefcase

Summary: I am a traveler and a writer, and this mouse fits easily into my ultra-portable laptop briefcase. When flat it is thinner than a Belkin tinymouse, and it is infinitely more convenient when arched. The transmitter can be left in port (the backside of the mouse is magnetized, but I wouldnt trust that for travel).
The wheel strip takes some getting used to. it does have a different texture from the LR buttons, but it does not stand out like a wheel. Lateral motion can be a bit sticky, I think because of the narrow and high vertical design. This might drive gamers crazy, but it is a minor problem for work. It does not have the luxury feel of a big mouse, but for serious travel it is the best available.