After reading a suggestion somewhere online I decided to pop the cover off of my 470 to see if the fan wasn't plugged in properly.
First of all, I was amazed at how easily the cover can be removed. It pops right off with no fuss and it doesn't even have any screws in it, yet its still tight. Very convenient for cleaning.
Second, I was surprised to see that the fan's plug actually wasn't seated properly. It was "in" but I doubt all four pins were making perfect contact all the time. With the card installed and heating up and cooling down all the time its definitely possible that it could lose a connection at times. Maybe the fan control pin was losing connection but the power was still there, which would basically let it run full bore.
I'm really hoping that this fixes the problem. A guy on the eVGA forums said that he had a 470 with a fan that wouldn't slow down and he had to RMA it. I really don't want to do that.