Introduction

If there was a Moore's Law for user interfaces, it would state that the number of computers per user will double every two years. In the past four decades, we have moved from many users sharing a single mainframe computer through command line interfaces, to a single user with a personal computer using a graphical user interface (GUI). Today, increasing numbers of users are surrounded by multiple ubiquitous computing devices, such as BlackBerries, PDAs, and cell phones. As our devices connect to a global wireless network, we become members of a 24-hour global society---one where we are always connected, and always on.