I tried, for the better part of the afternoon, to install a Rocketfish USB 3.0 PCI Express Card RF-P2USB3
I'm trying to do this because, for whatever reason, the USB3 cards that are available for the Mac are easily eight times more expensive than the commodity cards that are targeted at PCs. Of course, this cheap Rocketfish card doesn't come with Mac drivers, so it takes a little finagling to get it working, so try this at your own risk.
First I tried installing the USB3 driver from the MultiBeast "Hackintosh" toolkit, and installed the card. Rebooted my machine, and... grey screen and creepy repeating garbled startup chimes. Uhhh... that's not good.
Pulled the card out, and it still just hung on the grey screen. Fortunately, I knew which system extensions (a.k.a. "kexts") it had added, and I was able to Hold down ⌘-S to start in Single-User mode and manually go in and delete them before rebooting again. Back to normal. Whew.
I reinstalled the PCIe card just to make sure it wasn't the hardware itself, and the Mac Pro booted fine again, so it was off to try a different driver.
What worked: I installed this set of kexts from osx86.net including the DP3 files for Mountain Lion. After a reboot, drives connected to the USB3 card showed up, and seem to be working well. Unfortunately, it doesn't seem that I'm getting quite as high a transfer speed from the drives as I do when they're connected to my retina MacBook Pro (with native USB3 ports) --- maybe 3/4 the ideal speed --- but I'm happy to have a couple of extra USB ports, and a still-impressive speed boost when using USB3 drives.
Of course, since this is using a hacked third-party driver to enable the card, this may very well break with future updates of Mac OS X. Just know what you're getting into, if you try this.
Update: I may have spoken too soon. It seems the card will only mount an external drive when you start or reboot the machine with the drive already plugged in; I haven't found a solution that enables hot plugging yet. Still, it does work when you do it at startup, but not exactly convenient.