[mnutt]

The reality is most customers are not affected, and overall service uptime is highest anywhere around.

Unfortunately it's really impossible to say in this case, since they don't release numbers. Informally everyone I know with S3 buckets in US-Default had issues this morning.

As long as you build your application to be AZ-tolerant, you won't run into problems.

What you say about multiple AZs is true for EC2, but many other AWS services (especially EBS-backed ones) tend to go down across the entire region. If you're serious about availability, you really need to be in multiple regions.