This is very strange, my new Macbook pro does not show computers on the network or my Airport disk unless the firewall is turned off (any setting apart from "allow all incoming services" blocks the drives).
Tried to delete firewall prefs (com.apple.alf.plist) and it removes all firewall settings, but the drives are still blocked!
Running 10.5.2 with all updates, any ideas, I would rather like a firewall?