When's a screen not a screen? When it's made of thin air, we suppose. Much like this touchscreen, which is constructed from light and ultrasound vibrations alone.
Called HaptoMime, the screen uses reflective surfaces to create a holographic display. Then, the presence of someone's hand at the (non-existent) surface is detected using an infrared sensor, which causes ultrasound transducers to beam out high-frequency sound towards your fingertips. The sound causes the sensation of pressure, effectively making it feel like you're touching a real, solid surface.
The holographic images are high enough resolution to read Times New Roman font at size 6, and the video shows that it easily allows someone to play a toy keyboard. The applications are endless, but it seems like a particularly sensible solution for situations where the lack of physical touch would be beneficial, such as in kitchen or in a hospital operating theatre. [New Scientist]