Private key is stored in your browser, you fool. So if you migrate systems and fail to migrate those (I don't even know where they're stored for Safari...), you won't be able to reinstall your certificates, no matter how many times you download the certificates from CACert.
I'm smarter now though, certificates were made using firefox, and have been _backed up_ to a file. (keys encrypted with a good password, of course.)
...I should make sure I have backups of private keys for my servers. Ohshit...? -.-;;;