Bright side of News claims first-gen Intel P55 based motherboards will not feature SATA 6Gbps support as there are problems with reliability of the Marvell 88SE9123 SATA 6G chips:
Asus will not be putting them back on the boards until they Q&A teams are certain there will be no performance or stability issues. The Asus R&D teams are working on finding and correcting the unlisted issue. It is possible that the issue could be resolved soon enough that the Marvel chips would reappear on the P55 boards but it is unlikely.
The Extra 6Gb/s performance would have been a nice feature on these boards but, in the end if they are not stable or cannot perform properly then it is best to leave them off for now. As to the rumors that it is a performance issue, this could be the issue but it is unlikely. Usually the only thing that would warrant pulling a ship this late in the game would be stability.
We spoke with Gigabyte and they are also removing the Marvel SATA 6G chips from their P55 boards. This would seem to confirm a larger problem than just a driver or performance issue.