For the last couple of years, I have had to use Mozilla or Firefox to connect to the online indexes to which my university restricts access -- because Safari didn't support connecting to proxy servers that used .pac files.
Maybe this functionality appeared sooner, but on a lark today, I went into Safari's prefs and clicked the Proxies: Change Settings... button in the Advanced tab. Doing so opened the Network preference pane, where the Configure Proxies pop-up menu included the option of "Using a PAC file." Simply entering the URL for the file in the "PAC File URL" box and clicking Apply meant that I no longer had to switch browsers to access restricted resources. Cool!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050606152720687