Be aware that a circuit diagram doesn't necessarily reflect the physical implementation. (Compare the circuit diagram for the Arduino Duemilanove at http://arduino.cc/en/uploads/Main/arduino-duemilanove-schematic.pdf to the physical board if you're in doubt that that statement is true. ;))
I agree with Grumpy_Mike that the physical pin that's separated from the others is most likely to be the common, but a multimeter would indeed confirm that, as darudude says.
Also, @CSingleton, there are only four terminals. Let's label them A through D. In the lower left corner of the datasheet (the circuit diagram of the connections), terminal C is represented as an oval, and it is filled in, probably to indicate it is the common. The solid line connects the arrows at A+C, the thin dashed line connects the arrows at B+C and the thick dashed line connects the arrows at D+C (well, C+D, but same thing).