I contacted Buffalo about the possibility of USB 3.0 support in Macs, and unfortunately the supplied PCI-e card will not work in OS X. An email response I received from Buffalo states the following:
Unfortunately we're only supporting Windows with our PCI Express Card as we're using an NEC based controller and there are no Mac drivers available for it at the moment. It might change in the future, but as it's out of our hands - as we don't create the drivers for this piece of hardware - we can't guarantee it.
Let's hope NEC and other manufacturers start supporting OS X soon, or at least have Apple supply a generic USB 3.0 driver. Meanwhile, people should still be able to use the PCI-e card by installing Boot Camp.