As noted in our forums, Apple's latest OS X 10.8.3 beta seeded to developers as build 12D76 yesterday contains new drivers for NVIDIA's high-end Quadro K5000 graphics card. NVIDIA announced the Quadro K5000 for the Mac Pro last September, but has yet to actually release the product.
The first developer build of OS X 10.8.3 seeded last November added AMD Radeon 7000-series drivers, so it appears that the public release of the operating system update will include support for several high-end graphics options. Just yesterday, we noted that Sapphire was previewing its Radeon HD 7950 Mac Edition graphics card at CeBIT.
Apple has not yet revealed when it will be releasing OS X 10.8.3 to the public, but we've heard that the latest build was distributed to company employees as a pre-release version, so it seems that its debut may be very close after over three months of developer testing.
Apple is expected to release a revamped Mac Pro sometime this year, although it is unclear exactly when the company is planning for the workstation to make its debut.