Verizon posted a "how to" – to help customer with the mandatory system update to Android (Froyo) version 2.2 for DROID by Motorola:
Reserving Power for System Update: "Install now" and "Restart &install" options willn't be available if your device doesn't have enough power availability. System update requires 40% or more power availability if not connected to an external power source and 20% power availability if connected to a power source.
System Update Instructions: » You'll be notified on you device of system update. Press "Install now" and your device will power off &back on and installation process begin. Note: If you choose "Install later" this'll delay installation, you'll be taken back to home screen, and message will appear every 30 minutes until you install » Pressing "More Info..." get you more information regarding system update. » After receiving first notification of system update, you can manually initiate by going to "Home" > "Menu" > "Settings" > "About phone" > "System updates". Press "Restart &install". Your device will power off &back on and installation begin.
System Update while You Are Roaming: Mandatory System Update: DROID by Motorola