Today my Arduino Due came in (my first Arduino), but I'm unable to upload sketches to it. The device "Arduino Due" is simply not listed under "Boards". I tried this with recent versions on Ubuntu and Mac. The USB device is available, and two LEDS light up on the board when connected. I /think/ it is a genuine board. At least it says it's genuine 
And the price was also genuine...
You don't say which version of IDE you are using?
Something like this, maybe:
http://downloads.arduino.cc/arduino-1.5.2-macosx.zip
Sorry for bumping this thread, but I just wanted to state for the record that I used a 1.0 release version of the Arduino IDE earlier, and that is not compatible with Arduino Due. You need a 1.5 beta. This is stated half way the downloads page for the IDE, but that is easily overlooked for someone who already downloaded and installed the IDE. So it is good to be documented on at least one other place... 