An ideal device will be lightweight, connect to any device or service (large public screens, small personal screens, etc.) and expose a standard keyboard interface as well as a full model of the hand, so that people could write their own software to allow the use of new gestures.
Having wrist issues, I’m especially excited about being able to type on varying curved surfaces, which should do a ton to alleviate repetitive strain.
About six years later, it looks like these things are on the horizon. Check ’em out!
The Kitty Project uses the opening and closing of standard electrical circuits to build data input:

The SenseBoard uses an inertial measuring unit to wrap around the wrist yet measure the motion of the fingers with two degrees of freedom:

The winner will be a product that can be worn at all times and allows for easy touch typing as well as gestures.