SSH Agent GUI for ssh-agent and related utilities: [link|http://www.phil.uu.nl/~xges/ssh/|http://www.phil.uu.nl/~xges/ssh/]
Check "Make Agent Global"
This might also be helpful (somewhere in the middle of the page):
[link|http://pirlwww.lpl.arizona.edu/user_notes/user_notes.cgi?id=86|http://pirlwww.lpl.a...r_notes.cgi?id=86]
Possible alternate route (Tiger only): [link|http://developer.apple.com/documentation/Darwin/Reference/ManPages/man8/launchd.8.html|man launchd]
Have no first hand experience with any of the above so I hope it gets you where you want to be.