I've noticed a couple times now that my 99 A8 will roll backwards in Drive with the motor runing while sitting on my sloped driveway. Only seems to do it after the trans is warmed up (like after driving the 10 miles home from work). Won't do it when cold, like when first started.

Anything to be significantly concerned about?

Here's the slope of the driveway.
<img src="http://i47.photobucket.com/albums/f174/DavesData2/Audis/DSCN5647.jpg" border="0" alt="Photobucket">

In my case the motor is running and the car is in drive.

slopes the car does not move. I guess Audi has some sort of hold feature depending on the slope of the ascent/decent.

higher rpm = more drag by the torque converter.

Those extra ~2K RPM when a cold car is started are enough to provide that extra bit of "hold" in the TC to keep your car from rolling...