Weblog Tools Collection has documented the get_avatar() function present in WordPress 2.5 so that I don’t have to. Big thanks. I can scrap the avatar post I haven’t got around to writing and instead answer some of the questions that popped up in the comments to the WTC post.
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.
In addition to accepting an email address or user ID, get_avatar() accepts an entire comment object. This is a convenience for using get_avatar() in the comments loop.
The avatar support in 2.5 does not attempt to add avatars into your existing themes. Themes must be written to use get_avatar(). Dynamic addition of avatars is left to plugins.
Avatars are displayed in several places in the admin. They will show up when managing comments, for example.
Display of avatars can be completely turned off via a preference. When turned off, no attempt is made to connect to an avatar service.