WARNING:
I just received 4 Due R3 boards, presumably genuine, not clones,
and two of the four had visible solder splatter on the legs of
the uP chip that shorted adjacent pins together.
Lesson, CAREFULLY inspect each board BEFORE applying power.
I tested 3 of the 4 boards (one went to a friend before I
had a chance to test it), and the first programming port
connected to Comm Port 29 (after the driver was installed)
and the second to Port 30, and both worked as expected
with a variation of the Blink sketch.
The third connected to Port 31, and Arduino 1.5.2 kept saying
that the port was busy, and other objections, but unable to
Upload, or even unable to recognize that Port 31 was available.
I used the Device Manager on Vista to re-assign to Port 16,
and things went fine. Unknown why.