An OpenBSD developer by the name of Ted Unangst recently wrote a post entitled USB 3 back compat in which he refers to how it is difficult or impossible to use USB 3.0 ports if the operating system does not yet have an XHCI driver. (OpenBSD lacks an XHCI driver at this time.)
The confusion partially stems from there being multiple different XHCI host controllers. The ones by ASMedia and other third parties do not function at all without an XHCI driver. The new Ivy Bridge chipset native Intel USB 3.0 controller is a different story.
On my Asus P8Z77-I Deluxe Mini-ITX board there is a BIOS option to set the Intel USB 3.0 XHCI controller to several different modes. As long as legacy USB is still enabled those ports usually function just fine as USB 2.0 ports. Although this varies from operating system to operating system, the default setting of Smart Auto may work just fine for USB 2.0 only operation if there is no XHCI driver taking control of the USB 3.0 ports. If Smart Auto doesn’t work, try the other three XHCI settings and see if one works for you.
Watch out for the Sandy Bridge motherboards with USB 3.0 ports because they are always a third party controller that does not operate as USB 2.0.