On WordPress.com right now, and in the next release of self hosted WordPress – version 2.6 - you have a few extra settings that relate to avatars:
What’s cool is that for people who haven’t yet added an avatar, like my friend Rama, instead of the “mystery man” icon, I am now showing an .
What’s an Identicon you ask ? “An Identicon is a visual representation of a hash value, usually of the IP address, serving to identify a user of a computer system” ( source Wikipedia ). In addition, the Identicon will show the same generated avatar each time for that person without an avatar.
I’ve turned it on on my blog, and you can see it in action in this post. If you blog on WordPress.com, you can access these new option in Dashboard > Settings > Discussion.
More info posted by Matt over on the WordPress.com blog.