Was it so quick that you didn't hear or feel anything from the
transmission/engine?  If so, probably no problem.  As the others say,
if it actually did engage reverse, it might or might not have caused
damage.  If it did engage, or try to engage, that would be putting
extreme and unusual load on parts like the gears and clutches in the
tranny.  My guess is that the clutches would take the worst of
it.  The final drive seems so heavy, that it would probably weather
it.  Seems like the design of the TC would be such that it would not
feed so much force back to the engine that it would try to turn
backwards.  I'm rambling...  (My dad did this in a 1980 Toyota
Corolla and it stalled the engine and locked the rear wheels but
after the incident, there was no permanent damage to the car.)
Edward

At 06:22 AM 5/1/2008, Mark C wrote:
>If you are driving down the freeway, and decide to move the lever
>from drive to 2nd, and get confused, and by bad-habit depress the
>button, and slip into neutral and then into reverse, for just a
>split second, what happens?
>
>A. No harm, the torque converter et... et... what-ever-one-would-say...
>B. The Final Drive gets cooked. Howling is soon to follow.
>C. The Transmission gets a fresh influx of metal shavings.
>D. That money you have in the bank? Get ready to spend it.
>F. Nothing happens. Life goes on, and so does your Vanagon.