I had SshKeychain installed before and am trying to get back 10.5’s ssh agent to work. I found this article which pointed me to the solution and this fascinating file
From How to get Leopard ssh agent to work:

The solution is to use terminal, go to your home directory, cd to .MacOSX and look and see if there’s an environment.plist file. In there will be some XML to set this persistent string for SSH_AUTH_SOCK. You need to take that out. If there’s other stuff in the file, like a CVSHOME entry, hand-edit the XML to take out the SSH_AUTH_SOCK entry. (How to do that is beyond the scope of this post.) If the only thing is that entry, just delete the environment.plist file.”

And this is an even better article that describes the the past situation, how it works now, and the integration with launchd