Like its predecessor, USB 3.0 will offer backward compatibility.
USB 3.0 cables are now 9-wired, instead of the previous 4, and have modified connectors.
Dimensions remain the same. USB 3.0 functionalities are provided when the added signal lines become connected.
USB 3.0 connector
USB3.0 Standard-A Connector
USB3.0 B Connector
USB3.0 Micro-B Connector
The connector has been modified to maintain connectivity with existing USB devices and cables.