Talk about serendipity…
I was just talking about using EC2 and S3 for backups and along comes a solution to actually make it easier to build.
Without persistent storage, it would be hard to persist the backed up data to S3 as you'd have to do the file to S3 mapping (I think) by yourself. With this feature, you effectively create a blob of S3 storage and use it as a drive in your EC2 machine instance.
Actually, that's how I thought it worked all along, but was weirded out earlier today when I found out that it didn't.
Well, as of 9pm tonight it does.