While most upcoming P55 motherboards from ASUS will not offer built-in SATA 6Gbps support, the company plans to offer an optional SATA 6Gbps add-on card. The card features a PLX8613 expansion bridge chip and will provide you with two SATA 6Gbps ports, but it is unknown how much it will cost.
The only motherboard that will have built-in SATA 6Gbps functionality is the ASUS P7P55D Premium, this high-end motherboard will offer two SATA 6Gbps ports as well as the standard six SATA 3Gbps ports. Hexus was told that this functionality isn't offered on all P55 motherboards because SATA 6Gbps is still a relatively expensive niche feature.
There's certainly a performance improvement to be had, though. ASUS' P7P55D Premium test bed, equipped with a Seagate ST3500410AS drive, scored a HD Tach Burst Speed of 281.5MB/s. Compared to a SATA 3G board - scoring 185.9MB/s - that's a notable boost.