You could also just go into the SSHKeychain preferences and check the box that says 'Manage (and modify) global environment variables', so that it will set $SSH_AUTH_SOCK for you.

Erm. No. The whole point is that the way that SSHKeychain manages the environment (via the  .MacOSX/environment.plist in your home directory) doesn't propagate to some things like cron which are independent of your login.