A lot of retail sites experienced problems with high volume on Black Friday and Cyber Monday.
When a site puts up their "temporarily unavailable page" why not capture email? I think consumers would find it very convenient to get notified when you site is available. Sure everyone wishes it was never down in the first place, but at least don't make them keep checking if you're up. Tell them with email.
Of course if you do this, you'll want to be clear whether you are asking permission to notify them when the site is available, or to give them a lifetime subscription to your email. I think most consumers will expect the former.