Contrary to other articles based on earlier Palm devices, you do not hold the power button to hard reset the Palm Treo Pro.

1) Remove the battery

2) Hold down the red End key

3) Insert the battery

Update: here's a much easier method for hard resetting the Treo Pro. Just soft reset it with the stylus or by software, and when the screen is out and just before you feel the battery vibe, hold the red End key until you see the screen below. You do not have to remove the battery.

You'll see a screen like this (if you get the Palm progress bar you've done it wrong):

Press the UP-key on the d-pad to erase all data and restore factory settings.