I performed an automatic update to WordPress 3.1.1 and no errors were reported. After the upgrade, I noticed that I was no longer able to perform routine administrative tasks. I first noticed the problem when I tried to upload images to my media gallery. The upload appears to work alright, and I can see the thumbnail of the right image in the gallery - but when I try to insert the image into a post I just get a link to the attachment, which results in a 404 error. If I try to edit the properties of the image in the media gallery, I get a "You are not allowed to edit this attachment." error.
It seems like my administrative privileges are all messed up or something. Similar things happen when I try to add a new user as an administrator - I get a "Cheatin' uh?" error.