After installing Apple’s 2008-005 security update on my Leopard machines I found that ssh-agent was no longer being started on system boot. I use
ssh with key-based authentication for lots of things and not having it automatically loaded and communicating with my keychain was a real pain. After monkeying around with lots of ways to get it going again I found the following command, entered in a terminal and followed by a reboot, did the trick:
On OS X all roads lead to launchd.