I've upgraded to 3.1, but now am stuck in a loop when I try to access the back end.
I get the "Database update required" screen, then when I press the update WordPress database button I get a new screen with just the WordPress logo; no success or failure message.
The URL of that screen is
Any attempt to access any back end page takes me back to "Database update required"
I don't think my database has been upgraded because the version in the wp_options table is shown as 15477.
FWIW the site appears to be running fine from a visitor's point of view, but it's a bit limiting to not be able to post anything ;)