The main change in the Arduino Diecimila is that it can be reset from the computer, without the need to physically press the reset button on the board. The Diecimila uses a low dropout voltage regulator which lowers the board's power consumption when powered by an external supply (AC/DC adapter or battery). A resettable polyfuse protects your computer's USB ports from shorts and surges. It also provides pin headers for the reset line and for 3.3V. There is a built-in LED on pin 13. Some blue Diecimila boards say "Prototype - Limited Edition" but are in fact fully-tested production boards (the actual prototypes are red). More