Hi Scott Williams,
Thank you for contacting us.
At the moment, there is no such plugin or control, but it can be implemented. I will let you know about the steps:
1. Override the <sfWeb:CommentItem> control, which is used in the ContentViewSingleItem.ascx template.
2. Subscribe to the ItemDataBound event. In this event, you will access the IPost item, which holds the user's e-mail.
3. Place an image in the overridden CommentItem control, and assign the path to the src attribute:
<img src= "https://secure.gravatar.com/xmlrpc?user=[email_hash]" />
Where the email_hash is the MD5 of the user's e-mail.
I hope this makes sense, but I should say that I like the idea pretty much. Perhaps we can have a blog post about this implementation.
the Telerik team