Proxy Configuration Via "proxy.pac"
Internet Explorer Caching - Microsoft Internet Explorer's automatic proxy caching; how it can interfere with proxy.pac load balancing and failover
Most modern graphical Web Browsers implement APC, including the current releases of Firefox, Internet Explorer, Netscape, Opera, and Safari.
Sun's Java Web Start (JWS) has limited support for APC.
Manu Garg has written a PAC-parser library with python bindings, available at http://code.google.com/p/pacparser/
Original specification: http://wp.netscape.com/eng/mozilla/2.0/relnotes/demo/proxy-live.html
Wikipedia overview: http://en.wikipedia.org/wiki/Proxy_auto-config
Microsoft proposed IPv6 extensions to proxy.pac: http://blogs.msdn.com/wndp/articles/IPV6_PAC_Extensions_v0_9.aspx