I don't mean to sound snarky, but it seems like you don't deal with customers very often. You're thinking rationally when you should be thinking emotionally. Customers are hugely inconvenienced by downtime, but there's nothing to be done about it. You just need to get them to calm down and remember that you're someone they like doing business with, and this is just an unfortunate mistake that's outside of your control. Like everyone else has already mentioned, everyone knows that websites go down, so customers can deal with it as long as you give them some peace of mind[1].

Here's a related anecdote: it's common for potential customers to ask me about security before signing up. I used to actually answer them by explaining the details of our security practices, but no one understood what I was talking about. Now I tell people that the site is hosted on Amazon's servers so we can take advantage of the infrastructure they've built. Obviously this is a non-answer, but it makes people feel a lot better. They knew they wouldn't be able to evaluate our security anyway, they just wanted some sign that they can trust us (and they already trusted us because we're the only company that picks up the phone when they call).

So I guess what I'm saying is that when you're dealing with technology, it's good to be rational. When you're dealing with people, emotions are what matter.

[1] Just so you know, one other thing that I would say during the downtimes was that we were in the process of switching from Rackspace to Amazon precisely because of these problems. We weren't just sitting around accepting them. Since making the switch, the service has been rock solid, so customers know we meant what we said.