Note that the hardware can be set up with one sPin and several resistors and rPin's for calls to various capacitive sensors. See the example sketch.
Grounding and other known issues
The grounding of the Arduino board is very important in capacitive sensing. The board needs to have some connection to ground, even if this is not a low-impedance path such as a wire attached to a water pipe.
Capacitive sensing has some quirks with laptops unconnected to mains power. The laptop itself tends to become sensitive and bringing a hand near the laptop will change the returned values.
Connecting the charging cord to the laptop will usually be enough to get things working correctly. Connecting the Arduino ground to an earth ground (for example, a water pipe) could be another solution.
Another solution that seems to have worked well on at least one installation, is to run a foil ground plane under the sensor foil (insulated by plastic, paper, etc.), and connected by a wire to ground. This worked really well to stabilize sensor values and also seemed to dramatically increase sensor sensitivity.
Scroll Wheels (well, slide pots anyway)