John Thomson  managed to build a pressure sensor to behave like a switch, when it’s in an idle state the LED is off, if you apply some pressure,  light goes on:

The pressure pad doesn’t just work straight out the box – it requires a little bit of craft. What I’ve done is sandwich a pressure sensitive conductive sheet – known as Velostat – between two pieces of felt. I then stitched some conductive thread through each piece of felt – this applies a current to the pad and when the sandwich is put together, the circuit is complete. The Velostat acts like a resistor – the value changes when pressure is applied. It’s then just a case of writing out code that tells the LED to come on when the pressure reading goes over a certain threshold.

Take a look at the first and  second part of the experiment on his blog.