I've read about overcoming the max persistent connections of a browser by using various subdomains and also by using various ports.
Do browsers treat example.com and www.example.com as different hosts? This would obviously be the quick win for many who developer websites with a large number of images, css files and javascript files.
Yes, they are different hosts and browsers treat them as such. It is just the common practice that usually one redirects to another but that is only a practice, not the rule.