OK, this isn't much of a hint, but ... after I upgraded my iMac to Panther, I received an error when I tried to change my firewall settings through the sharing control panel that stated "other firewall software running." I have seen this posted to other forums as well. To resolve this issue, just delete Library -> Preferences -> com.apple.sharing.firewall.plist and reboot.
Boom, just like that things work again.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003102320312464