In the not too distant future, we should have screens that will "blister" to represent on screen buttons. The tech exists now, it's just not quite ready for use outside of a research lab yet.
Longer term, I'd expect we'll be making use of the doppler effect. The device will project a keyboard onto pretty much any surface you point it at, and will be able to tell what key you hit based on the echo your fingers make hitting the surface with the keyboard projected on it. Again this already largely exists, but it would require a very precise microphone to be accurate, not to mention even a basic projector bulb for the keyboard would mean that we probably need some kind of advancement in battery tech or a much more efficient overall system.
Long term, what I'd like to see, is a setup like what they created for the TV show Earth: Final Conflict. Essentially you have a sort of universal portable phone/PDA, and it has a rollable display (also something that exists today). So you sort of pull it apart like ancient roman/greek scrolls with the rollers on both ends. There would be one setting for just normal operation, and then you could pull it out into an "extended" mode to have a keyboard.