Option to force identicons

This isn't too easy. You have to provide the e-mail address MD5 hash in the Gravatar API URL, and there's no way to 'override' and not display the Gravatar if they have it. I see only two potential options. I guess you could provide some kind of transformation of the hash, but you still can't guarantee that somehow it will match another e-mail address of an undesired user. If you use a randomization of the hash, you won't end up with the same avatar for the same person/e-mail.

I guess probably the most valid solution would be to port the WP-Identicon module to Drupal. If you're interested, you can contact me for an estimate to do this.

For my part, I see 2 possibilities:
1. in the case of anonymous comments, add a checkbox (in the comment before, just below the e-mail) to allow the user to choose whether to use the gravatar (if is one of course). Then the image default Drupal displayed (if it exists).
2. in the case of authenticated users, they can choose whether or not to use their gravatar by changing the appropriate option in their account settings.

In 2 cases, this is not "identicon" to be used, but users have the choice of anonymity ...

I guess Dave's proposal would be more appropriate! :-P

Yeah, being able to do this for anonymous comments with this module would add another unwanted layer of complexity. If users don't want their Gravatars to display on a site they can use gmail-style e-mail addresses like myname+nogravatar@gmail.com or probably shouldn't be using the Gravatar service to begin with. :)

Yes, I'm totaly agree with you! :-)

Ok I'm going to mark this as 'by design' then.