There’s the occasional interest from people in proxy autodetection and autoconfiguration but for some weird reason the Squid documentation has never really touched on it.
This article seems to be quite a good FAQ relating to proxy autodetection/autoconfiguration with a lean towards “gotchas” that exist in various implementations. There’s also a few other places to eyeball: the original Netscape proposed standard for proxy.pac; The Wikipedia article and the “King” of PAC files which filters porn and spy sites (well, as best you can based on URL matching.)
There’s also the Microsoft WPAD for IPv6 extension – the specification and a developers’ blog entry covering the background.
I’ll try to wrap all of this up into a Squid Wiki article or two so there’s at least a single source of information for everyone to use when deploying proxy configuration and autodetection with Squid.
The Squid Web Proxy/Cache Blog 