I can login to StackOverflow but not Meta, and vice versa.
I've cleared all my cookies etc but absolutely no joy and I've associated all my accounts (didn't know if that had anything to do with it).
I did have a hunt around meta.stackoverflow to see if there were similar problems.
I'm running IE7 on Vista Ultimate. This problem also happens on my other machine running Windows 2008 Standard SP1 and IE7.