yeahdixon: apparently we screwed up and put the old NG bootloader on some of the Diecimila boards. You'll need to press the reset button before uploading your sketch. Or you can burn the Diecimila bootloader, but you'll need an AVRISP mkII or other hardware programmer.
Can you upload a program to the board from Windows or Linux using just a serial cable (no usb-to-serial converters)? Some USB-to-serial (and possibly your GeeThree serial port) don't work with Arduino serial boards, presumably because they don't implement some particular feature in the way avrdude or libusb expects them too.