WordPress.com use Amazon S3 for storing images, they do not serve images directly from Amazon, they use Varnish reverse proxy at front end, so not all images need to be served from S3, Varnish cache popular images and serve directly
Many large web sites use reverse proxy servers in front end. For example flicker use squid in reverse proxy mode, wordpress use varnish in front end. Varnish was written from the ground up to be a high performance caching reverse
