Lynnfield (somewhat erroneously labeled as Core i5) and the accompanying P55 chipset (codenamed Ibex Peak) are meant to reduce the cost of building a Nehalem system, but trouble with Marvell's SATA 6GB controller will leave motherboard vendors with less room than they typically like when differentiating product SKUs and prices
The problem lies within the Marvell 88SE9123 controller. Multiple motherboard vendors, including ASUS and Gigabyte, have demo'd early P55 motherboards with this chip onboard, but further testing has revealed that the controller isn't delivering anything near the performance it should be. At this juncture, a number of motherboard companies have opted to pull the controller completely off the board and plan to reintroduce it when the relevant issues in software and hardware have been hammered out.
The Gigabyte GA-P55-UD4P, one of the affected boards.
In the end, I don't expect that a lack of SATA 6GB support will have much of an impact on mainstream-priced Nehalem uptake. The various motherboard vendors that were counting on Marvell for a solution probably aren't real thrilled at having to change their product designs this close to launch, but it'll be months before we see actual 6GB devices on the market. Traditional hard drives, meanwhile, probably won't benefit all that much from an additional 3GBps of burst potential. Solid state flash drives may have more room to shine, but SSD performance is itself impacted by the OS in use, the storage controller built into the drive, and what firmware version is installed. The number of additional variables make it difficult to gauge how much of a real-world performance boost the new 6GB standard will deliver.