I've had a search around, but I can't find a definitive answer.. does Apple keep the open source stuff such as Apache patched with security updates using the normal software update system, or do I need to handle it some other way?
Since Leopard is the newest the bugs and such in it will be youngest. As time goes on the unpatched vulnerabilities and such will just get older until either they patch them, which is unlikely, or they ask you to upgrade to Snow Leopard.
If it's not facing the outside world, i.e. intranet-only, then it's probably not as big a deal. If it is a big deal you might want to just install Xcode and maintain your own patches based on stable releases of Apache. It's not too tough.