I had the same vexing problem with an arduino NG board that was reported by others, with it not starting when the board is powered on external power (9V through the power supply from Sparkfun), but all running fine when powered through the USB plug.
I was relieved when someone pointed me to the "10k resistor between RX and GND" fix, which I just put in. Except... the problem persists. The board still will not start the program when I have only the external power plugged in, but it runs fine if I have the USB plugged in and connected to a computer (even if it's powered through the external supply).
The resistor is running from the pin where RX (and 0) is marked on the end of one long black connector to the GND pin that is the second to last pin on the opposite end of the other black connector. Is that the way it's supposed to be connected?
Are there any other secret little tricks one needs to know to get the board to work?
Any and all help is much appreciated.