"High Availability Offering — For developers and business who want additional resilience beyond the automated backups provided by Amazon RDS at no additional charge. With the high availability offer, developers and business can easily and cost-effectively provision synchronously replicated DB Instances in multiple availability zones (AZ’s), to protect against failure within a single location."
Amazon's 'replication' is not mysql replication though, it is a drbd block level replication and can not be used for scaling reads like normal mysql slaves can be used.
The app I've been working on doesn't use replicas for read scaling, just cache-money (ergo, activerecord). We're partitioning for write scaling but no slaves (well, we're doing master/master replication but not for read capacity).
After having had to contend with replication lag, lots of instances spinning and related headaches for years I'm pretty convinced that scaling reads with binlogs is in my past for good; it's way more expensive than having a good write-through cache.
"High Availability Offering — For developers and business who want additional resilience beyond the automated backups provided by Amazon RDS at no additional charge. With the high availability offer, developers and business can easily and cost-effectively provision synchronously replicated DB Instances in multiple availability zones (AZ’s), to protect against failure within a single location."