PeterH:
Do you have anything at all connected to the Arduino except the USB lead to the PC? If not, I'd be looking for errors in the way the sketch was uploaded or the Arduino was powered.Have you tried commenting the Serial.begin() out, verifying and uploading and just prove that it then starts blinking as normal?
nothing at all connected to the arduino,
if i comment out or remove the Serial.begin line then it uploads and runs fine
also if i add to the void loop: Serial.println("test");
i get "test" in the debug window every second, but the LED is not flashing. so that proves that the arduino is running and hasn't locked up. removing all serial references proves that the LED is working.
i have no idea what could be causing this or why it would happen on two different arduinos, but it seems to be the cause of hours of bug hunting.