You’ll probably be updating to Mac OS X 10.5.1 anyway, if you’re running Leopard, but note that this update contains three fixes for Leopard’s firewall. (I’ve discussed some of the weaknesses in the firewall here and here.) This update patches some bugs in Leopard, so you should certainly install it. As usual for Apple updates, check the Software Update preference pane in System Preferences. You can find out more about the security fixes on this Apple security page.
