Seems that WordPress v2.5 (which will be out this month) will include support for Gravatars by default:
Theme Authors: Adding Gravatars to Your Theme
The function to add Gravatars to your theme is called: get_avatar. The function returns a complete tag of the Avatar.
The function get_avatar is setup as follows:
function get_avatar( $id_or_email, $size = '64', $default = '' )
* id_or_email: The authorâ€™s User ID (an integer or string) or an E-mail Address (a string)
* size: The size of the Avatar to display (max is 80).
* default: The absolute location of the default Avatar.
That’s the default avatar icon up there. Ugh. I am really not interested in gravatars, I am a fan of Monster ID and Wavatars. I hope Scott and Shamus can update their plugins to hook into the native 2.5 functionality as that would be a lot simpler. Adding a dropdown to the Admin panel to let you select between different icon sets is probably the best approach.
UPDATE: Ryan Boren says that any avatar service can be invoked, not just Gravatar:
Gravatar is the service used by default. get_avatar() is completely pluggable, however, so any service can be used. get_avatar() is built-in so that themes will have some fixed API on which they can rely, regardless of whatever avatar service is being used behind-the-scenes.