By popular demand, we've reduced the minimum TTL (Time To Live) for Amazon CloudFront from 24 hours down to just one hour. This means that you can now use CloudFront to distribute content that changes from hour to hour.
As has always been the case, you will set the expiration time using the Cache-Control, Pragma, or Expires headers, per RFC 2616. Note that you will set these values on your Amazon S3 objects as S3 Metadata. S3 sends the metadata to CloudFront in the form of HTTP headers.You may want to read Mark Nottingham's Caching Tutorial to learn more about the ins and outs of caching.
Most of the popular S3 tools and toolkits give you the ability to set custom headers when you upload objects to S3.