A USB FAQ says
Will my existing peripherals still work? How will they co-exist?
The good news is that USB 3.0 has been carefully planned from the start to peacefully co-exist with USB 2.0. First of all, while USB 3.0 specifies new physical connections and thus new cables to take advantage of the higher speed capability of the new protocol, the connector itself remains the same rectangular shape with the four USB 2.0 contacts in the exact same location as before. Five new connections to carry receive and transitted data independently are present on USB 3.0 cables and only come into contact when mated with a proper SuperSpeed USB connection.
Since USB has proven itself to be the overwhelming favorite interface, a crucial pillar of the USB 3.0 specification addresses how the SuperSpeed interface must remain compatible with the millions of existing USB computers and devices in the marketplace. For the most part, it's good news: all USB 2.0 devices will work on USB 3.0 hardware, and (nearly) all USB 3.0 devices will work on USB 2.0 hardware.
The USB Implementers Forum (who develop the USB spec) say
SuperSpeed USB is backwards compatible with USB 2.0. Devices interoperate with USB 2.0 platforms. Hosts support USB 2.0 legacy devices.
All USB 3.0 ports should work with USB 2.0 peripherals.