One thing about Cayman that might be deceiving. Its NOT that 6800 series replace 5700 and 6900 replace 6800. Its more that 6700 will succeed 5700, and 5800 "split" in a bit lower 6800 and a bit higher 6900. I predict following:
a) Cayman will be priced a lot higher then 58xx series - probably around 500$ mark not 400.
b) Cayman is NOT "double barts" the way 5870 was a "double 5770". Dont expect it to be 100% faster. It has 40% more shaders, and even more importantly the 6xxx series seem to be bottlenecked a bit by memory (as overclocking tests show). So given exactly same bus width in cayman, unless it really sports rumored 6Ghz memory (i seriously doubt it, although 5+Ghz is a strong possibility), i wouldnt even count on 40% increase in performance. Hopefully im wrong and lets make it 50% - that will most likely put Cayman at about same performance level as gtx580.
So in the end, i expect pretty similiar performance from both companies for about same price.
Atm i would go with nvidia cause of CUDA/PhysX (i plan on buying 2x580 or 2x6970 or 6990 so ill have enough ports for 3 displays anyway), but of course ill wait a bit.
Why just "a bit" ? Because the whole delay im afraid my turn into few months , if AMD decides to respond to gtx580 and "buff" 6970 to sport 384 or so bit memory bus and maybe more shaders, to actually steal performance crown.