There is a large difference between the audience for standalone GPUs and ready to use laptops.
nVidia customers tend to look for information and research ATI vs nVidia. Most of Apple's customers are wowed by the user experience (which still has not aged), which cannot be compared. nVidia's sales would fall dramatically if they become outdated in terms of hardware. Apple's sales tend to stay the same (or in our case, rise).
Apple has nothing to worry about. A few spec whores jumping ship is nothing. The main audience is still there buying MBPs.
People who are clueless are still buying the old hardware. People who absolutely need a mac right now are also buying the old hardware. But a LOT of people are holding out, and some are even moving on.
I agree that they don't have to worry right now, and the much improved user experience has always been Apple's greatest selling point. Mac OS and the (generally) excellent quality and support have always justified the higher price tag on Apple computers.
But that being said, this is a very delicate situation. Take a look at the average buyer's point of view. They've bought a Mac because they wanted one and were clueless (or didn't care) about specs. Apple has made a sale. But now this customer sees what other companies offer at a price to hardware ratio (from a friend, flyer, etc.) and compare it to what they just spent. When Apple's hardware is equal to the competition's, the price disparity is much smaller. But at a point like this, the gap is rather huge. This customer who was happy with their Apple purchase is now questioning whether the extra price was really worth it. Now next time they go to buy a computer, they may not automatically choose Apple, so customer retention goes out the window.
TLDR: Once people realize how much they paid for outdated technology, they're going to be pissed.