proxy.pac file issues in Leopard
Ah! I'm glad you mentioned this issue. I've been fighting with something similar for a while now on my Macbook Pro.
At my workplace, I have to configure it to use a proxy server, and set it up to use a proxy.pac file to automatically get the proper configuration.
Under OS X Tiger on my laptop, having the option to use the proxy.pac file didn't seem to create problems when I was on other networks. OS X would apparently try to get the proxy.pac file at the proxy address given, fail to find one, and default back to running without a proxy server.
Ever since upgrading to Leopard though, my laptop was only working properly on my network at work. When I came home or used someone else's wireless network, I was able to ping addresses just fine from the terminal, but browsers like Firefox or Safari wouldn't pull up any pages. (Actually, if I let it sit long enough, it'd *finally* pull up a page - but we're talking over an HOUR to do it!) Widgets and "Software Update" wouldn't work either.
(Interestingly though, my copy of Windows XP Pro running under Parallels desktop WOULD connect to the net just fine under these same circumstances.)
I was finally able to get my laptop to stop using the proxy.pac file though, and then it worked fine on my home wi-fi network again. I honestly can't remember exactly what I did. I do remember seeing issues where I'd simply de-select the option to use it, and after I saved my settings, it was selected again. I think I just blanked out the line that had the path to my proxy.pac file and left it selected, using a blank entry?
On another note, shouldn't this be something you can configure by location? EG. If I create a new network "location" and call it "work", why can't I key in a proxy.pac file for that location, but have it default to not using one on the other, "Automatic" location? I *think* I tried this and the proxy settings seemed to persist across multiple "locations".
I figured it out for those interested. There is a bug in Leopard that when you change the web proxy to use a PAC file, you cannot change it back. I told Apple customer support about the bug and they have submitted it to engineering. The workaround is to delete the preferences file and then reboot.