For those of you poor souls out there in Mac-land that still rely on a dial-up connection to surf the net, be sure your IPv6 is turned off! I struggled for many hours trying to establish a connection when I realized the error. The IPv6 is located in System Preferences -> Internal modem -> TCP/IP (located in the tool bar between PPP and Proxies). Good Luck!
[robg adds: This is the first I've heard of this -- can someone in modem-land confirm this one?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040518100306781