thanks for all the replies/confirmations of the problem. I'm also curious if you've had this problem personally in other cars.
anywho, I just got back an email from them with some more information.
They see it on gentle declines to much steeper declines. By gentle decline, they mentioned a local road that drops *maybe* 100 feet over a mile for a while - ie, not very steep.
They don't have any problem with it maintaining the correct speed going uphill, even on steeper roads.