Andrew Tsu of touchscreen-maker Synaptics on the interaction and usability considerations inherent in designing for touchscreens:
Unlike mechanical controls, which designers have to take into consideration during the hardware design phase, a touchscreen shifts the planning of device controls into the software design phase. This shift can certainly improve the user experience, since designers can implement the layout with fewer hardware constraints. However, the shift could also lead to real disasters in usability.
The obvious example is inconsistent behavior of controls across applications — imagine how confusing it would be if a touchscreen phone required users to single-tap the virtual keypad buttons to dial phone numbers but to double-tap on the virtual buttons in their music player application.