(Prelude: If you're getting this error message, some program other than the Mac OS X firewall control panel has created firewall rules using ipfw. You can check whether or not this is occurring by issuing the command
sudo ipfw list
in the terminal. If you're sure that you don't want the firewall rules shown, then
sudo ipfw flush
and delete /Library/Preferences/com.apple.sharing.firewall.plist. This should reset your firewall settings.)
Finally had time today to run down some really irritating behavior that had shown up in Mac OS X lately - whenever the Powerbook went to sleep, it would forget and then never reacquire the DHCP configuration it got from the network. Reboot and the problem would be solved, but who wants to reboot every time you wake up your laptop?!
Hunted around and found that the most common cause for this is a firewall getting in the way of the DHCP interaction. Trouble is, I've never installed firewall software beyond Apple's built in stuff. Head over to System Preferences, sharing, firewall to check it out and got the dialog "Other Firewall Software is Running on your Computer". What the heck is this? "sudo ipfw list" shows that TCP ports 990, 999, 5678, and 5679 all have special rules associated with them. Huh, I didn't do that. Turns out those ports are for ActiveSync.
Then I remembered PocketMac. I bought a great new cellphone (Audiovox SMT 5600) a little while ago which rocks except that, because it's based on Mobile Windows, it needs software that can synchronize with the Mac's contacts, calendaring, etc. So I bought PocketMac, fiddled with it to get Bluetooth syncing working and never looked back. Only problem is that the software installs firewall rules for you that f*** up DHCP. Went to system settings, accounts, login items and deleted PocketMacStatus.app from the list of startup items. Reboot, sleep, wakeup, no worries. *sigh*.