Thunderbolt, with its bi-directional speed of 10 Gbps, is currently the fastest peripheral connection available in the Mac and many PCs. However, if the USB 3.0 Promoter Group is successful at ratifying the protocol, a new Super Speed USB 3 standard will push USB to the same 10 Gbps speed.
Of course, the new Super Speed USB 3 protocol probably won't arrive anytime soon; if adopted, it may not see the light of day until 2014, at the earliest. But the trade group was confident enough to make an announcement at CES (the Consumer Electronics Show), and provide a USB-Futures PDF that outlines the protocol and the review process.
Apple was slow in adopting USB 3, and I don't see it being in too much of a hurry to adopt Super Speed USB 3, at least not while it's championing the Thunderbolt interconnect system. However, both protocols have unique advantages that complement each other, so if Super Speed USB 3 does make it to the market, Apple will likely include the port on some future Mac.