Storing passphrase in keychain
$ ssh-add --apple-use-keychain ~/.ssh/billmill.org.key
Enter passphrase for /Users/llimllib/.ssh/billmill.org.key:
Identity added: /Users/llimllib/.ssh/billmill.org.key (key for billmill.org)
that's it! now you can just use that key and ssh will grab the passphrase from keychain.
From here, with a useful caveat:
(If this fails, make sure you are using Apple's version of
/usr/bin/ssh-addand not something installed with brew etc.; check with
I missed step two, which is to add it to the ssh config file
...any amount of IdentityFiles you want ot use keychain for...