Here is the relevant part in the HTTP/1.1 spec:
"Clients that use persistent connections SHOULD limit the number of simultaneous connections that they maintain to a given server. A single-user client SHOULD NOT maintain more than 2 connections with any server or proxy. A proxy SHOULD use up to 2*N connections to another server or proxy, where N is the number of simultaneously active users. These guidelines are intended to improve HTTP response times and avoid congestion."
By hosting static content (images, stylesheets whathaveyou) on multiple hostnames (like using a CDN for example) you can avoid congestion caused by fetching the components 2 at a time. A little something everyone should know!