I have OpenID support enabled by wpopenid/WP-OpenID+ on my blog. Also, I use the Gravatars plugin to show avatars for each comment writer.
I had problems with Gravatars not showing up for OpenID commenters. After an hour or two debugging I found the problem and a working fix that required two lines of gravatars2.php to be changed. I've posted it on my blog (naturally).
Hope this might help someone in the future.