After a few searches, here's what I found (read the steps once before attempting, some of them are time dependent).
- Start the car with your foot on the brake as usual.
- Press the ODO button until "ODO" appears on the dash display. (If it's already present, cycle around once to display it again.)
- Turn the car off.
- Again with your foot on the brake, press POWER to restart your Prius.
- Within 6 seconds of starting, press and hold the ODO button for 10 seconds and don't release it.
- You're still holding the ODO button after 10 seconds, right? Now shift the car into Reverse and quickly press the Park button. You may now release the ODO button.
- The odometer display should now show "b on" (if not, start over).
- Press the ODO button until the display shows "b off."
- Turn the car off by pressing the POWER button.
- Start your Prius now, put it reverse and enjoy the silence.
Note: This is supposed to work only with US models, so if you're abroad it's up to the kindness of your dealer. And I've only tested this on a 2009 Prius, so your mileage may vary :)