My Leopard SSH agent was not working after upgrading a Macbook Pro. I was using a few other applications in the past to manage my SSH keys – in fact, I didn’t even know this was a feature until the new install on a Mac Pro worked without me installing the ‘keychain’ app from fink, or a number of other solutions that I had tried in the past.
To fix this, I had to edit the ~/.MacOSX/environment.plist file. In that file, there is an entry for SSH_AUTH_SOCK which needs to be removed. I just removed the the section that looked like:
<key>SSH_AUTH_SOCK</key> <string>/tmp/503/SSHKeychain.sock</string>
The text may differ slightly on your computer.