Is there any merit to the idea of certificate viewing software showing a gravatar/identicon for the hash of an SSL certificate, to aid in identifying it?
Yes. At least with OpenSSH this has been done. I don't see why it wouldn't work with SSL as well. But I know of no implementation.
OpenSSH calls these images "RandomArt", "SSH Fingerprint ASCII Visualisation", "Visual fingerprint", "Visual Host Key". They have been there since version 5.1 from 2008. But I've only ever seen these images when creating a new key. Not when actually using one with ssh. (And that's because it's off by default on all ssh clients I've come across. You need the -o VisualHostKey=yes option.)
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