http://aws.typepad.com/aws/2011/12/amazon-s3-object-expiration.html
S3’s new Object Expiration function allows you to define rules to schedule the removal of your objects after a pre-defined time period. The rules are specified in the Lifecycle Configuration policy that you apply to a bucket. You can update this policy through the S3 API or from the AWS Management Console.
This is a great new feature for S3 so you don’t end up with some endless clutter of files that you really don’t need. I can imagine using it for expiring tings like archived log files or backups.