You can see today that there is clearly more power behind the HD 4890. So, should ATI have increased the core clocks and memory clock on the cards further? - Yes and no really. For starters, we don't know what the yield rate on a 950MHz core would be; it could significantly drop the rate and increase the price of the card.
Secondly, by moving to these clocks there would be no room for OC models and we know how much companies love to do them. We have already seen GIGABYTE increase the core and memory clock; albeit, not this high.
Where ATI could have probably increased the clock, though, was with the memory. The 975MHz is very weird; they should have just gone for the straight 1000MHz clock. The other thing about having the card clocked higher would have been the difference between the card beating the GTX 260 more often and at a higher rate than its current scores against it.
With everything said and done, though, there seems to be some serious room for overclocking and once we see companies move to aftermarket cooling, there should be even more room again.
It will be exciting to see what companies like Sapphire do with TOXIC and ATOMIC models, while ASUS mix it up with the TOP series and HIS bring out an IceQ4 version with some new clocks.
What's going to be even more interesting is when we slap two of the overclocked cards together. Performance should be extremely strong and the setup should become quite attractive to a lot of people.
ATI ultimately have a really good product here and like I've said before, while it's not next generation, the name doesn't suggest it ever was going to be. It really is a HD 4870 on steroids and while we would have loved more, the price point of the card and the performance it can offer makes it a great card for people.