The university I work at uses its DHCP servers to update a dynamic DNS server when you connect to their wireless network. Sometimes the DNS update gets lost in the shuffle, and you wind up with an IP address but no DNS entry. The solution to the problem is to release your DHCP address and get a new one -- but the Network control panel in Panther only has a "DHCP renew" button (not the same as "release").
I found a message on the web that describes one way around this:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040330141701462