AWS EC2 promises 4 9's (4.3 minutes of downtime/month) before their SLA kicks in, but they only give a 10% discount until availability dips below 99% (7.5 hours of downtime/month) when they give a 30% discount. If availability is below 95% (36 hours) in a month, they give a full refund.
For an individual instance, they only promise 90% availability.
Availability of network access to existing instances.
What you're talking about with provisioning capacity is a totally different matter. Provisioning availability is not guaranteed (unless you purchase reserved instances) and there are frequently periods where certain instance types are not available in certain AZs, though they do try to resolve that as fast as practicality allows them to. It really stinks sometimes though - especially if you get into a situation where something fails in your autoscaling group and there is no capacity available for a replacement instance. Usually you can get around that though by making sure your ASG is set up for multiple AZs, or worst case changing instance types (though that can be problematic in it's own way).
Yeah, you can now provision multiple instance types in an ASG which mitigates this somewhat.
I think people sometimes forget that the cloud isn't magic, and a sudden burst of requests for new instances needs somebody to actually rack up some servers.
Availability of running instances, I don't think they make any guarantees for availability unreserved on-demand instances (I don't see how they could).
For an individual instance, they only promise 90% availability.