As we noted in our review of the 790GX chipset, AMD's SB750 south bridge has a somewhat unusual feature: Advanced Clock Calibration. In short, ACC allows the chipset to connect to spare pins on the Phenom processor in order to tweak the CPU for overclocking. And it works. We were able to overclock our Phenom X4 9580 from 2.5GHz to 3GHz using ACC, while the chip would only hit 2.7GHz with ACC disabled.
This feature is currently exclusive to boards with SB750 south bridge chips, but the folks at Expreview have nabbed a presentation slide that suggests Nvidia will add the same feature to its mobos soon. In fact, the slide claims Nvidia MCP72 and MCP73 chips already "fully support" the ACC tech. If I'm not mistaken, the MCP72 goes in nForce 780a and 750a motherboards, while the MCP78 is synonymous with the nForce 730a and 720a.
The slide goes on to say the feature "requires board modifications" to work, so current nForce mobos apparently can't perform the ACC voodoo. However, Expreview claims Nvidia is working to have MCP72 motherboards with ACC support ready next month.