Just a quick tip. If you use more than one id_rsa key for sshing around the place, you can add the second key into your keychain with something like:
ssh-add -K ~/.ssh/my_other_key
Be careful if you have the macports version of openssh installed as well. The -K option is specific to Apple’s ssh-add.
Dave Dribin’s blog has a decent discussion about the weird ssh-agent stuff that goes on in Leopard. Mostly I find it annoying, but I suppose Leopard is trying to be helpful.