I'm hitting this also, just felt I should report in.
On one of my forums and this one (XF), it shows an avatar I haven't used in six weeks on Gravatar--I put this one in place at the beginning of college football season. On another forum, it shows the one I changed to later in December, but none of the other forums have seen it change yet (including here). I switched the Gravatar a few days ago yet again, and
none of the XF forums' Gravatars have changed.
And hate to say it but with any other site or service I use that uses Gravatar, those are updating properly, some within a few minutes, but just about all within 24 hours.
This is also just a recent problem. Earlier in 2011, Gravatars would usually be changed in all the forums I've accessed, usually being updated all over within 24 hours, just like I've seen on other sites.
I have tried changing to a static avatar or deleting them entirely, and even have re-entered a different email address (different Gravatar account) and then changed it back...old Gravatars still load. My next step is to go in and start deleting avatars from my Gravatar account to see if I can force a change that way.
Something isn't updating. I wish they could be contacted to sort this out. They're run by Automattic Inc., aren't they? Maybe hunt down Matt Mullenweg and get him to point us to a fix?
It does seem like an erratic caching issue on their end, especially since this worked earlier in 2011.