Is using gravatar a security risk?
A recent Slashdot article speaks about how somebody was able to discover the email addresses of 10% of SO users through the username and Gravatar system.
"Gravatar offers a global avatar service, using an MD5 hash of the user's email as avatar ID. This piece of information in some cases is enough to retrieve the original email address. Testing a simple attack on stackoverflow.com, I was able to determine the email addresses of more than 10% of the site's users.