Identicons would be great for named anonymous, if they were based on something the user voluntarily entered, like the email address. That way it's not difficult to change if the picture generated is really ugly.

For other anonymous uses, I can see how the identicon would make the conversation easier to follow in those conversations where the commenters would sign their name anyway, but I'm concerned that it would be a barrier to commenting for most anonymous discussions, especially if the identicon would be the same across journals and comms. So it might not be a very used feature.