I don't think this can be a definitive statement yet. AMD has not lifted the NDA on the details of the architecture yet so there's not a lot of "official" details. However, ExtremeTech did have the article up prematurely for a little while before they realized it and took it down. While I didn't get the opportunity of reading the entire article, it does mention that the memory interface has undergone significant changes. The ring bus found in r600 is completely gone in favor of spreading the interface around the edges of the chip. If we're talking about a completely new memory interface, that alters the core quite a bit.
I'm still thinking the drivers aren't quite ready yet, and it's was already said that drivers are responsible for the delay of the 4870X2 (until August). I would assume that the HD4850 would have been included in this driver delay as well, but ATI found that it was performing well enough to compete with Nvidia's offerings (which it was). Either way, we'll find out more info in a week or two once the NDA is lifted.
On the other hand, the 9800GTX+ hasn't undergone as substantial changes and the current driver should be a much better representation of future performance than the current ATI driver.