I had AWS customers in mind, more than Amazon.com themselves.

If you are renting instances in a specific data center, and you wish to have cross-cluster redundancy, it is your responsibility to build that into your distributed system.

Exactly. It's easy, though, to build something on top of AWS (or Rackspace or whatever) and then stand back, marvel at it's magnificence; and forget that an outage - in a network segment or service that you don't control - can totally wreck your masterpiece.