Yikes, we had a some client's tanks with no power but luckily two of them had a generator to hook up to. We are not sure how long the power was out on the other tank, but everything seems to have pulled through with only minor stress. Its a good reason to stock the tanks lightly!
If people cant afford one of those back-up power supplies, I suggest hunting down some battery-powered air pumps. I know you can get them at Petcetra. They are noisy as hell but will provide airation and flow to a tank in time of need.
-Diana