I've wondered and written before about the 'downside' or 'risk' of building your application based on one of the excellent SAAS services from Amazon. And I do think they are excellent. But check this post Amazon Web Services Gets Another Hiccup from TechCrunch:
"Amazon does not guarantee 100 percent uptime for its Web Services, although it does strive to achieve that. And data centers go down all the time, no matter who is hosting your data.But more and more companies are relying on Amazon to be able to scale their computing resources on demand and do it cheaply by paying only for what they need. Many Web startups are building their entire businesses on top of Amazon’s Web Services, and even an hour of unavailability is unacceptable. At least this one happened during the middle of the night."