Arduino SmartThing Shield Schematic?

Take at look at the detailed readme on my GitHub repo for my ST_Anything project. Should answer lots of questions and give you a jump start on integrating the Arduino/ThingShield with SmartThings.

To answer you question directly though - Pins 0/1 or 2/3 (switch selectable) are used for serial communications (usually 2/3) and pin 6 is reserved.