One of the new features we added in the Arduino Diecimila is the ability to upload sketches to your board without having to physically press the reset button. The Arduino software automatically resets the board before starting the upload. If you’ve got an Arduino NG or serial board, you can give it this same ability with a little bit of soldering. Tom’s written up the instructions for the NG, and nkcelectronics posted instructions for serial boards.

One cautionary note: because the automatic reset works by using the serial DTR hardware control line, boards will auto-reset whenever you open a serial connection (i.e. via the USB cable) to them. This should be fine in most cases, but you might want a second or two delay before sending data to the board.