You could well argue that the real innovation at Microsoft is going on at their Research arm, where the latest oddball idea is a tablet PC with a split keyboard stuck to the back. Each half of the 'board is rotated 90-degrees, so as to maintain the regular hand position. Dubbed "RearType", the layout is the product of a new research paper [PDF link] and is designed to allow for faster text entry without blocking screen visibility or relying on tactile-minimal on-screen keyboards.
It also allows for text-entry while standing or walking, without a stable surface to rest the tablet on. That way, rather than having to hold the tablet in one hand and peck at it with the other, users can hold their slate in both hands and maintain a speedy typing rate.
As for how fast that rate can be, when the Microsoft Research team gave the RearType prototypes to twelve skilled typists, after an hour's training the users had achieved speeds of fifteen words-per-minute: slower than a regular desktop keyboard, but on a par with a touchscreen 'board. The ergonomic issues of strapping buttons to the back of a slate are still to be figured out, but all it takes is an imaginative OEM.