I know that this will probably not be implemented, but I'll ask anyway - if in addition to being able to turn Gravatar's off being able to upload a selection of default avatars users can choose from.
My users don't seem to know Gravatar, and I feel uncomfortable promoting other people/company services. It is a really cool one I admit, but they are not paying me for promoting Gravatar in my forum nor they have asked nor I feel I should do it if I don't need to.