In one of our earlier discussions about cloud, an astute reader pointed out that one "downside" of public cloud, especially one like AWS, is that they make very few guarantees about your instances. While the system as a whole has service level agreements (SLAs), your particular instance does not. To quote:
"If your instances go down you're going to have to deal with it"
The underlying assumption, of course, is that you have better control over the level of availability of your particular instances and their underlying hardware, especially scheduled maintenance, when you control the entire environment rather than leaving it to a cloud provider like Amazon or Rackspace.