I just noticed that the default Gravatar is not actually resolving for our site.
Not a big deal, except that in IE (IE 8 and earlier, at least) this means the placeholder with the big ugly "X" gets displayed.
I think the problem is this: I have set name and email to required for comments (Settings > Discussion > "Other comment settings").
Therefore, the email field is never blank, and the get_avatar function in /wp-includes/pluggable.php always tries to contact Gravatar for that account's image.
Not a problem yet, because Gravatar is provided with the info for rendering a default (set to "blank" in the discussion options), and that default image is in place on the webserver.
If I take the URL from the "src" attribute for the image directly from the page source after the comments have been rendered by the browser, and paste that URL directly into the address bar, Gravatar correctly redirects to my "blank" default image.
(Initially "&" in the query string were encoded using "amp;" I had to change each "amp;" to "&" to get Gravatar to resolve properly. Otherwise, I just got Gravatar's logo image. Understandable. So I changed that in the get_avatar function.)
So, my question, why is the img URL not resolving correctly when on the page in WordPress?