Defering passphrase entry with ssh-add

Hello,

Is there any way to invoke ssh-add so that it defers asking for the 
passphase for my private key until the key is actually required.  It 
seems like such an obvious option to have; it would permit me to have 
"eval $(ssh-agent) && ssh-add" in my .shrc, but not encumber me with 
having to enter a passhrase every time I ssh in, nor would it prevent 
unnattended logins.  Am I missing something obvious in the way the 
system is meant to work?  Am I just being lazy in not wanting to run 
ssh-add before the first time I use ssh?

This question applies to two versions,
OpenSSH_4.4p1, OpenSSL 0.9.8d 28 Sep 2006
OpenSSH_4.5p1 FreeBSD-20061110, OpenSSL 0.9.7e-p1 25 Oct 2004

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Regards,

Chris