I just found the Gravatar Signup Encouragement plugin, and think it is a great idea. However, I am having problems getting it to work correctly and am wondering if it is maybe not compatible with WordPress 3.1.
First, when I activated the plugin, I saw the following message in WordPress: "The plugin generated 295 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin."
Second, even though I have a Gravatar, the plugin thought I didn't and tried to encourage me to set one up.