I've been using this SSH Agent (featuring Puffy in businessman suit and a briefcase) for some time since Mac OS X Panther (10.3). It worked for a while on Leopard, until I believe 10.5.5 update broke it. It no longer can access the Keychain for passphrase.
It turns out that Leopard has native ssh agent Keychain support. I had to remember removing the SSH_AUTH_SOCK entry from my ~/.MacOSX/environment.plist file (I previously configured it to the former SSH Agent). After logging out and logging back in, it now points to the socket of the Leopard ssh agent. Running ssh-add -K (the -K option is Mac OS X specific) works.
Not using my former SSH Agent has the added benefit that my machine no longer has to start Rosetta when I login. SSH Agent was compiled for PowerPC, not Intel Mac.