The buzy bees at Microsoft's Research lab have created a concept of a tablet PC with a rear-mounted keyboard that can be used while you're holding the tablet with both hands. The prototype looks pretty awful though, and tests revealed skilled typists could only do 15 words per minute after training with the device for an hour, which is close to what can be achieved on a touchscreen keyboard.
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).