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 (live) implementation. There was a thesis from 2004 that showed an implementation of this for Mozilla, but I don't think it was more than a proof of concept. (See "Further reading" section.)
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.)
Further reading
- SU:What is randomart produced by ssh-keygen?
- Sec:How secure is Visual host key, and how is SHA2 converted into that representation?
- Hongxian Evelyn Tay, Carnegie Mellon University, 2004-05-03, CS Senior Honors Thesis: Visual Validation of SSL Certificates in the Mozilla Browser using Hash Images