Microsoft may be set to completely redefine the mobile device keyboard experience.
This new concept, spotted by Mary Jo Foley, is called RearType and provides a three row QWERTY keyboard configuration on the rear of a tablet PC. The benefit of this configuration is that it allows users to get the physical sensation of touching keys just as they would on a Laptop or Desktop PC. This prevents the keyboard from taking up valuable touch screen real estate.
This solution should allow for text-entry while walking as well (although that is not recommended). While this appears to be a great new solution for mobile computing, there are still some quirks to be worked out. They include the fact that there are currently no plans for commercial availability, and the keys can still be triggered when setting the device down (they need a lock down mechanism).
A potential solution for that problem may be to add a proximity sensor that will disable the keys upon setting the device down. With regard to typing speeds, Microsoft Research gave their prototypes for RearType to twelve individuals that are great at typing.
These skilled typists scored an average of 15 word-per-minute after training with the device for an hour. That puts the scores lower than those that can be achieved with a desktop keyboard and very close to those that can be achieved on a touch screen keyboard.