Steven,
I'm really not sure what's happening here. I just re-registered on your site. My current Facebook avatar was pulled in, as expected and mentioned above. I then logged out of your site, went to Facebook.com and updated my avatar there. When I re-logged back in to your site, my new avatar was pulled in, as expected.
We simply can't recreate this issue. Can you tell me:
1) Can you recreate this with multiple accounts, or is it one specific Facebook account?
2) If you try to register from a Facebook account, is the avatar imported correctly during registration? We use the same methods to import the photo on register and login, but it's possible a file permission on your existing avatar is set in such a way that the login import isn't being allowed to overwrite the existing one.
3) If there's any steps you can provide that seem to work consistently from at least 2 accounts, that would help (browser, OS, updating the photo, only on registration, etc).
Let me know, and we'll gladly help to investigate further. Sorry you're having issues, but it's simply not something we can recreate.. which makes it very difficult to diagnose and correct. Knowing how widespread the problem is with the above information will greatly help.
Thanks,
Alex