http://www.clarksys.com/blog/2012/03/02/howto-cache-s3-objects-with-varnish/

s3.vcl

backend s3 { .host = "s3.amazonaws.com"; .port = "80"; }

sub vcl_recv {
  if (req.url ~ ".(css|gif|ico|jpg|jpeg|js|png|swf|txt)$") {
    unset req.http.cookie;
    unset req.http.cache-control;
    unset req.http.pragma;
    unset req.http.expires;
    unset req.http.etag;
    unset req.http.X-Forwarded-For;

    set req.backend = s3;
    set req.http.host = "my_bucket.s3.amazonaws.com";

    lookup;
  }
}

sub vcl_fetch {
  unset obj.http.X-Amz-Id-2;
  unset obj.http.X-Amz-Meta-Group;
  unset obj.http.X-Amz-Meta-Owner;
  unset obj.http.X-Amz-Meta-Permissions;
  unset obj.http.X-Amz-Request-Id;

  set obj.ttl = 1w;
  set obj.grace = 30s;
}

http://www.ducea.com/2009/08/04/using-varnish-in-front-of-your-amazon-s3-static-content/