We have received a few complaints recently from folks who signed up on one of the sites to ask a question or post an answer and were surprised that their avatar was automatically pulled from Gravatar. In some cases it was because they forgot they even had a Gravatar account. In others it became a privacy issue - posting anonymously becomes more difficult if their Gravatar is a photo of themselves.
Now, we could just tell people to use a different email, but that seems like a poor experience all around considering that we recently rolled out an alternative to Gravatar.
So I suggest we change the default for new users to be a generic (either static or randomly generated) avatar and leave it up to them to switch to Gravatar if they're into that sort of thing.