Given that AWS are running the same software across regions and have the same people & processes in place, and further that there's software that runs across regions (e.g. S3), I'd wager it's not long before we have a multi-region outage.
Finally, some of the multi-AZ problems in the past were compounded because as one AZ went down everyone hammered the other AZs, taking out the APIs at least. That's when everyone believed that AZs were isolated. Now that people know that's not the case, those same systems are going to be hammering across multiple regions.
