The SATA 6G support is powered by the Marvell 9123 chipset and a high-speed PCIe bridge chip while the USB 3.0 is coming from an unknown add-on controller.
UPDATE: ASUS got back to me and told us that the USB 3.0 implementation on both of these motherboards uses the NEC UPD720200 chipset on a single PCIe 2.0 x1 lane that supports up to 5.0 Gb/s. Considering 4.8 Gb/s is the theoretical maximum speed for USB 3.0, this implementation should have all the bandwidth you could need.
This is the ASUS P6X58D Premium motherboard that implements both of these features for the LGA1366 Core i7 platform while including three PCIe x16 slots and six DDR3 memory slots. You can see the USB 3.0 ports on the back panel here (the blue ports).
The P7P55D-E Premium motherboard will be one more LGA1156 motherboard to come from ASUS with TONS of features. Besides the new SATA 6G and USB 3.0 support, it includes a 48-phase power solution and the ASUS instant-boot ExpressGate OS.
We are looking forward to seeing these motherboard cross our test bench and should have a brief preview on the first SATA 6G-ready hard drive this week!