Yes, USB 3.0 is backwards compatible. However, some new instructions are added to the new 3.0 standard so some functionality might not work. However, most hardware will support USB 2.0 and I won’t expect too much problems with an external hard drive.
Reference from PCWorld:
The beauty of USB 3.0 is its backward
compatibility with USB 2.0; you need a
new cable and new host adapter (or,
one of the Asus or Gigabyte
motherboards that supports USB 3.0) to
achieve USB 3.0, but you can still use
the device on a USB 2.0 port and
achieve typical USB 2.0 performance.
Yes, the devices will work together at USB 2.0 speeds.
It depends on your device. A hard drive sould work but obviously not with full speed.