Is this useful? I guess it could be, rather than the myriad of web presences and individual icons that every site uses, to have one default icon used everywhere.
Once you’ve registered on http://www.gravatar.com/ you can upload assorted image icons, these can then be referenced from anywhere via a URL based on the MD5 hash of the email address:
echo -n email@example.com|md5sum
See also: WPMonsterID :: Dammit Jim!