AT90USBKey with Arduino environment

I'd like to use the AT90USB1287 as a much more powerful chip than the various Arduino variants, and one which has native USB, so I don't have to spend money on an otherwise useless TTL cable. The AT90USBKey demo board has lots of features and much cheaper than building my own, so this seems the best way to go (only $30).

What do I have to do to enable native USB download for this board under the Arduino environ? (I want it to run a Repstrap 3d printer, and don't want to have to rewrite it under AVR Studio). Thanks David