The backward compatability issue is an interesting one. It could be that future cars, 2004 or 2005, require GF-4 oils and cannot use GF-3 oils. If GF-4 oils are only 5W-20 or 0W-10 or whatever, it'll make for some interesting commentary. And, some folks will insist on using the wrong oils and blame the car makers for the future failures. Other folks will just be ignorant and use the wrong oils. Of course, only time will tell if the new oils are suitable for engine longevity--it seems that the car makers are only interested in 100,000 miles to cover the extended warranties they sell, and many consumers still think that 100k is an adequate life for an engine.
Ken