Look at this description of using the Qprox Capacitive Touch Sensors
http://www.tigoe.net/pcomp/code/archives/arduino/000331.shtml#000331
For those applications where you want to sense a person touching an object, but can't use a switch or a force-sensitive resistor, charge-transfer touch sensors can often be very useful. The one used in this example is made by Quantum, and it's very simple to use.
Regards,
David