sensitive capacative surface sounds about right.
It's not that hard to build a basic single point "touch sensor" from some metal foil and an a/d-converter that will react long before you actually touch it.
I'd imagine the hard part would be to filter out unwanted signals and get resolution and reliability up.
I've also seen this done with a desk-sized screen with cameras behind a back-projected screen interpreting the shadows of your hand.
This screen, which we ported some of our games for to make this demo:
(though we don't use the near touch feature in our demo)