Thanks to an extension for Thunderbird called Display Contact Photo, you can now view Gravatars within emails in Thunderbird. The Gravatar blog has a good explanation on how to setup and install the extension. I’ve installed the extension as Thunderbird is my email client of choice. So don’t be surprised to see a beer mug accompanying replies from me although I’m not sure yet if both parties need to be running the extension to see the images.
Been using a similar plugin for about a year now, works well also but the extension site has been dead for about 8 months.