Let's look at the current core frequencies:
4850: 625 MHz;
4870: 750 MHz.
As you can know, almost every hardware vendor striving to release its own designed cards, and the non-reference frequencies on them are not so seldom. So why not to let AMD/ATI to make more money by releasing another models of RV770?
Let's do some calculations

750 (as 4870) / 625 (as 4850) = 1.2 times = 120%. What about these new models?:
4860 = 687.5 MHz (110%);
4880 = 812.5 MHz (130%);
4890 = 875.0 MHz (140%).
Of course, those new frequencies should not to be exactly (say -5%/+5%). Of course, the models with higher frequencies (4880, 4890) will be much more rare than ones with lower, but they will cost more money for the consumer and make more profit for the vendor!
You see that nVidia will release GeForce GTX 270 (yes, a middle of 260 and 280) very soon, and, IMHO, it will be a clever decision to breed more 48xx. Don't you think?
Seems that TSMC's 40 nm technology has not so good yield
. So why haven't you, guy, release 4890X2? I understand that this model is very complicated one to design, but who knows when TSMC's 40 nm will be fine? Who knows how long should you wait to produce RV870 in large quantity?
About Catalyst. You, guys, are capable to design the state-of-art chips like RV7xx, but, alas, Catalyst's quality is not perfect. I understand that it is extremely difficult to support several driver versions (I mean: WinXP-32, WinXP-64, WinVista-32, WinVista-64, Win7-32, Win7-64 and lots of Linux/Unix), but you should definitely do something. Do everything you want, hire for Terri Makedon's team the best specialists in the world (I know, it will costs some money), but, please, make Catalyst perfect (I mean: no graphics issues and superb performance). I think it is shame to release so named Catalyst Hotfixes every time. Perhaps, you should change Catalyst's monthly schedule to 2 or even 3 months. Please, remember what was happened with S3 and Matrox - both of them are outsiders now because of nasty drivers.
BTW, please, emphasize gamer's attention on Radeon's Edge-detect CFAA. This technology makes PC games' visuals pretty appeal, so tell gamers about it everywhere you can, at least at AMD Game.