I would think it would be fairly easy to add. There is already image processing in the message list and the message detail page. AquaMail pulls the image from our contacts if it exists. But adding Gravatar support would put a picture in when none can be pulled from the contacts. It is a free API that I use on websites and it seems pretty popular. It is used on other mail clients that I have seen and I enjoy it a lot. Maybe if you create a new contact from an email and the Gravatar icon has been pulled, that could go into the contact too. And of course if the user overrides the mage in the contacts, that override should remain the master image.
There is documentation on using it here...
http://en.gravatar.com/site/implement/hash/ [nofollow]http://en.gravatar.com/site/implement/images/ [nofollow]Just a nice little feature to add if possible.