http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1230286016
Unlike an interpreting microcontroller (like a basic stamp), there is no particular overhead on a per-statement basis, and it can be difficult to predict exactly how long a given "sketch" statement will take to execute. However, the basic instruction cycle of the underlying CPU is 62.5 ns (16 Mips), and simple statements (like "byte i = 0;") take just about that long to execute.