Is this true? I've been mining with reference 5970s and 6970s at 100% for the past 3 months and just received my first reference 5870s from Ben's last week. They've been running at 100% for a week with no issues. Is there a defect in the 5870 fan design that isn't present in 5970s and 6970s?
There is no defect in the fan design and there is no defect in the card. The problem is simply that you don't want to let the rotating assembly of the fan get very hot. As long as the card is staying cool, you'll be fine... although you'll obviously be putting unnecessary wear on the fan and wasting a small amount of electricity (5 to 9 watts).
If you're running a card that actually *needs* 80%+ fan speed to keep cool (very hot inlet air), the fan will be trashed in no time. You'd need to clean out the old grease and apply some very high temperature bearing grease to deal with that sort of abuse.
I wish I still had the old fans. I'd take post some pictures or make a video. I think I tossed them though.