Great article from RightScale on an implementation of master/slave replication using Ec2/S3.
Not a lot of details, but a great high level explanation of what you want to accomplish.
What we’ve built is a mysql master/slave set-up with backup to Amazon S3. The set-up consists of one mysql master server which is the primary database server used by the application. We assume it runs on its own EC2 instance but it could probably share the instance with the application. We install LVM (linux volume manager) on the /mnt partition and place the database files there. We use LVM snapshots to back up the database to S3, this means that we get a consistent backup of the database files with only a sub-second hiccup to the database.