Brent Sordyl's blog

You can now configure an Amazon S3 bucket to bill the requester, rather than you (the bucket owner), for both request and bandwidth fees associated with access to the bucket. Requests against such buckets must be authenticated and bear a header that signifies consent to be billed for the request. Bandwidth between EC2 and S3 within a region remains free of charge when this option is enabled.
(Link: Amazon Web Services – Announcing “Requester Pays” Option for S3)