From the concept shots and information leaks, both Microsoft and Apple's feature on-screen inputs for doing a lot of things, but they don't talk about text input. Is it going to be a virtual keyboard? Probably. But that doesn't preclude a USB port for a physical keyboard.
What would you use? Obviously, if you need to take this around town, on the train or in the car, you'd need to use the onscreen keyboard. But what about at home and at work? Which would you prefer to use? And why?