Sony has currently under development (http://www.theinquirer.net/?article=9991) a new LCD screen that will let you feel and push virtual buttons, PDAs are said to be the first products to use the technology although this would open the possibilities to virtual keyboards, etc...

The technology uses piezo effect vibrations and Sony has cracked the problem of the high voltages normally required. The piezo effect can be subtly altered to create a number of different vibratory effects, allowing humans to get the sensation that their fingers are actually touching real buttons on a screen.