Smart environments, wearable computers, and ubiquitous computing in general are thought to be the coming `fourth generation' of computing and information technology [1,2,3]. Because these devices will be everywhere -- clothes, home, car, and office, their economic impact and cultural significance are expected to dwarf previous generations of computing. At a minimum, they are among the most exciting and economically important research areas in information technology and computer science.
However, before this new generation of computing can be widely deployed we must invent new methods of interaction that don't require a keyboard or mouse -- there will be too many small computers to instruct them all individually. To win wide consumer acceptance such interactions must be friendly and personalized (no one likes being treated like just another cog in a machine!), which implies that next-generation interfaces will be aware of the people in their immediate environment and at a minimum know who they are.