I couldn't decide where else to put it, so I settled on miscellaneous.
After I upgraded to 2.5, I had gravatars working on my site. After upgrading to 2.5.1, I can see the gravatars in the main comments administration screen, but on the posts themselves everybody gets the default "no avatar" icon.
I double-checked the settings, which had changed from permitting PG icons to G. That shouldn't have mattered, but I put it back anyway. Gravatars were and are enabled, though.