I second this. While EC2 takes a bunch of really hard problems, allowing you to provision virtual servers at a moment's notice, the value proposition for RDS is much more marginal. It's simply not that hard to get MySQL up and running, even in a RAIDed configuration, and the fact that the underlying store is EBS means most of the advantage of RAID is spent simply overcoming the I/O handicaps of EBS.

We prefer running our MySQL instances on ephemeral storage, which is physically closer to the instance, and deal with I/O volumes by using lots of replication.