All of a sudden, comments started appearing with no gravatars.
It happened sometime in the middle of this post (http://www.doseofdigital.com/2009/12/7-biggest-mistakes-relationship-marketing/#comments). Every comment before this is fine, every one after this has the gravatar missing.
I noticed that the image that makes up the gravatar in the ones that are working are served from a URL like this: http://www.gravatar.com/avatar/77de7fff550531b5d8e2f12d41ca56e7?s=32&d=identicon&r=PG
Where those that are missing are served from one like this: http://btspiurl.appspot.com/clsnola75
Obviously, this change in source is the problem, but I have no idea how this happened.
UPDATE: looking at this further, it appears to be a problem with comments served via backtype. They are all tweets. These used to work fine, but don't any more.