Managed DNS like this is interesting. We keep a Slicehost account around because it's DNS manager is top-notch. When the command-line tools get better for this, it might be a good alternative.
Here's where this will rock: if I could sign up for S3, Route 53 and Cloudfront, then deploy a site entirely hosted on AWS. Use Route 53 to point the A records of my domain directly at the Cloudfront distribution and the MX records at Google Apps. Is this possible?