The chip will be smaller, cooler and cheaper to produce. It will probably have a higher clockspeed too.
Samples are scheduled for May, while production should start in Q3 between June and August. Such timing clearly indicates that ATI is not in a rush to release the R600 and we know that R600 is actually doing just fine. The R600 needs Vista to glow.
We heard about the R580+ before, but it was called R581. Obviously ATI changed its mind about the name. The card naturally supports all that R580 can bear, including Avivo, Crossfire and it is a native PCIe component.