I have now included gravatars on this site. Not that I get many commenters to use them *sniff*....
I've added a comment below to show how they appear. If you don't have a gravatar then you'll get a default icon (I have plans afoot to automatically assign these should the commenter not have a suitable gravatar set - but first I have to generate a few icons).
Clicking on any gravatar will take you to the gravatar website.
Feel free to experiment to see how your comment will appear.
Update: I have added Identicons for those without Gravatars