I recently re-installed Lion on my MBP and ran into the problem that by default ssh-agent remembers my private key indefinitely. I wanted to reduce this to something sane and found the file /System/Library/LaunchAgents/org.openbsd.ssh-agent.plist which controls ssh-agent's behaviour. I started tweaking stuff with launchctl, first broke my ssh-agent altogether and after a while figured out how to tweak it to my needs.

After initially breaking my ssh-agent because I copy/pasted commands that I didn't really understand, I found the following apt quote:

A good rule for rocket experimenters to follow is this: always assume that it will explodeAstronautics, issue 38, October 1937 (source)

Fortunately I managed to fix it and get some basic understandign of launcd on OSX. This information seemed to be a bit fragmented, so here are my findings.