Amazon just added NAT to their EC2 service. It also sounds like they will be turning the old direct addressing scheme off soon. This is probably a step towards assigning static IPs to your hosts in EC2. It may even allow you to have EC2 instances with no external IP address at all. It makes sense to not use an external IP when all you are doing is processing data from S3 and then sticking it back into S3. You can read more about the NAT change here.