I wrote a plugin which retrieves these gravatar images and assigns them to contacts in the OS X Address Book application. The disk image is . To install it, drag GetGravatar.bundle to Library/Address Book Plug-Ins in your home folder and restart the Address Book application.
Click on any email address to be offered the opportunity to download the corresponding gravatar. Be warned, it will replace any image assigned to that contact with their gravatar if they have one.