I received an email from Demitrious Kelly at Gravatar, who was nice enough to tell me that they are now supporting transparencies in their avatars again! This is part of their recent changes which have also broke Blogengine.NET support for Gravatars. Ah, well, nothing can be perfect. The fix for this is out there, btw.
You’ll need to visit the Gravatar site and reup your original, alpha blended PNG’s again.