You can request anyone's avatar with curl like this:
curl "http://gravatar.com/avatar/$(md5 -q -s firstname.lastname@example.org)?s=300" > avatar.png
Replacing email@example.com with your (or anyone else's) email address, and 300 with the pixel size (up to 2048). For example, s=300 will get you a 300x300 pixel image.
If you don't have access to a Unix command ...
Five years later, but you can now (and for quite some time) add additional email addresses to your Gravatar account. Just log in to your account, add the email address, click the link in the confirm email and you are done.
Still not as easy an practical as a "+" wildcard, but quite doable.