At a minimum, you should use at least two availability zones if you care about high-availability on AWS. The vast majority of issues (including the current EBS hiccup) tend to be limited to one AZ.

For us, today's outage triggered a bunch of alerts, knocked 1/4 of our USE attribution servers offline (we're in four zones there) but otherwise caused no impact. Mostly just acked alerts and twiddled thumbs waiting for situation to resolve.