When Buffalo released a number of USB 3.0 products a about a year ago, there was some confusion over whether or not the included USB 3.0 PCI-express card would work in Macs. If so this would have brought USB 3.0 connectivity to OS X far sooner than expected.
Unfortunately the card supplied by Buffalo did not work in OS X, though users could boot to Windows installations in Boot Camp to utilize the hardware for USB 3.0 performance.
A couple of weeks ago electronics developer CalDigit made a similar announcement, releasing a new AV Drive with USB 3.0 support. Along with the drive, CalDigit includes a card for supplying desktop computers with USB 3.0 ports. Unlike similar previous announcements from Buffalo, the new card from CalDigit comes with support for OS X, so if you have a Mac Pro you can get USB 3.0 connectivity on it.
Information on the card is available at the CalDigit website.
While this should work, CalDigit makes it clear that performance may differ on unsupported (non-CalDigit) USB 3.0 products. The company is clearly disclaiming any poor performance people may experience, but if you would like to try USB 3.0 on a Mac Pro system, at least one option is available.