Right now, the FAQ on Gravatars is pretty clear: they only associate email addresses with faces. I think it would rock, however, if URLs could be associated in a like manner. That way, when I send a TrackBack or something, my smiling mug would show up instead of whatever default image the implementation uses.
Of course, this is a fundamental shift in architecture, because now you’re trying to associate URLs with individuals, which is a dangerous proposition. Most email addresses map 1:1 with the people who own them [exceptions being catch-all collection points like email@example.com, etc.]; this is probably less true with URLs, although I can’t point you to anything that would prove this assertion in either direction.
[As Lara notes, she and I talked about this last night. I’d thought about this before, but our conversation sparked the writing of this suggestion.]