Hmmm, yes Fudzilla is also saying that, but I also read somewhere that they're on track, although pretending they're not so they can observe what the GTX 580 is capable of before shipping the final Bios with the final clocks.
BUT, Fudzilla also says that Cayman will have 1536 cores. Now, if the 1536 core number is correct, there is no plausible explanation for a manufacturing problem, given that the 5870 has 1600 cores and probably a bigger die size.
I've done a little speculative math based on that number. Here it goes for your consideration:
If we take into account the optimizations that Barts brought, where a 960 core GPU is equivalent to something like a 1340 core Cypress (a little slower than the 1440 Core HD 5850), and a 1120 core GPU is equivalent to a 1500 Cypress (slighly slower than the 1600 core HD 5870), then I could say that, on average, AMD has "gained" around 380 cores with their optimizations.
In that case, a 1536 core Cayman GPU will have more or less the performance equivalence of a hypothetical last generation 1980 core Cypress. And this makes sense, since this number, give or take, was circling around the net for a while. They just didn't take into account the performance optimizations where AMD has managed more for less.
Taking this into account, let's make a small (yet again speculative) comparison:
The 5850 has a 160 core difference to the 5870 (1440 -> 1600)
The GTX 470 has a 32 core difference to the GTX 480 (448 -> 480) and again another 32 core difference to a full 512 core Fermi.
Now, the GTX 480 is faster than the 5870.
And the GTX 470 is faster than the 5850, but slower than the 5870.
So, and given the differences in cores of these parts, one could say that:
448 Nvidia cores is faster than 1440 AMD cores. But an increase in 160 cores makes the HD 5870 faster.
If you were to give the 5870 another 160 cores, you would probably get GTX 480 performance or slightly better.
So, say 1600+160= 1760 cores.
But if Nvidia is to release a GTX 580 with the full 512 cores, then the distance would remain the same. Now, if you add another 160 cores to the AMD part 1600+160+160 = 1920, which would put them both competing on the same level.
But 160+160 = 320. Given that AMD has gained around 380 cores in optimization, it's possibly slightly better than Nvidia.
Now you have to factor in the fact that Nvidia may also have made improvements to their own architecture, which could make this more complicated.
But if they haven't, it might all be down to effective CPU and RAM clockspeed.
This is why AMD is probably trying to figure out how Nvidia is going to market their cards. If Amazon's pre-order page (that was removed since) is true, then it might just be a full 512 core affair and little else in respect to improvements.
I think this will be closer than we might think.