There is kind-of a warning, of sorts. Normally, users will either have the screen at full brightness (for daytime use) or dimmed (for night). When the 720 loses external power, the screen brightness immediately shifts to 50% brightness...so, during the day, you'll see the screen suddenly dim, and at night you'll see it brighten.
After several seconds, the screen will shut down, but the unit has not yet completely quit running. If power is applied very quickly after the screen goes black, you can still pick up where you left off. (When the screen goes dark, though, you're almost out of time...there might only be a 2-3 second window to restore power at that point.)