I can't remember where, but a while ago I read something about using subdomains to serve up a sites resources as a way to potentially speed up loading. The theory was that the protocol that browsers use to communicate with servers only allows some limited number of things to be download concurrently from a single domain (like 2 or 4?). But a site fairly commonly has dozens of resources. So if you were to create a subdomain (e.g. images.css-tricks.com) and use that to serve up images, that would be treated as a different domain and you would double the number of concurrent downloads possible.
In trying to research it, I haven't been able to turn up a lot of quality information. Some forum threads are condemning it saying that multiple DNS lookups would then be required slowing things down more than speeding things up. Others going to far as to say that Google may penalize for this (which seems relatively absurd).
I'm always trying to improve the efficiency and speed of my sites where I can. This past weekend I was trying to improve my CSS Sprites use on this site. This was the result. It is fairly trivial to create a subdomain, so I tossed it up on there. This is just one image so it doubt it will make any big difference, but I'll definitely look into moving all of the images from the theme onto a subdomain if there is any conclusive evidence this is a smart idea.
Anyone have any good information on this?