I just brought home the new Arduino UNO rev 3 from my local Radio Shack.
The installation was not easy or obvious. I am new to Windows 7, so that doesn’t help either.
I tried various forums with no answers that worked. This thread explains my problem:
Simply updating the driver with “Arduino UNO” on this Unknown Device did not work.
The device manager would continue to say no driver was found.

I the tried these 10 steps and it worked for me. Your mileage may vary.
Unknown Device
Unknown Device Found.

Update Driver

Update Driver.

Choose “…List of Drivers”.

Choose Ports

Choose “Ports”.

Choose “Arduino LLC” and then “Arduino UNO”.

Yes - Continue to Install the Driver

Yes – Continue to Install driver!

Driver Installed

Voila! Driver Installed.

Arduino UNO on COMx

Arduino UNO is now seen as a COM port.

Arduino UNO now in device manager

And now, yes, Arduino UNO shows up under Device Manager.

In Arduino IDE- Choose New COM port

Then, in the Arduino IDE, be sure to choose the new COM port!