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 from its cache.

Many big web sites use varnish or squid at front end as reverse proxy.

We’re now using S3 as the primary storage for WordPress.com, rather than just for backups. We have some layers in front of it, notably Varnish, so the majority of our serving doesn’t hit S3. Still, our AWS bill went from around $200/mo to $1500/mo, and rising. It has simplified some of our requirements, but doesn’t look like it’ll save any money.

http://ma.tt/2007/10/s3-news/