As happens every 3 or 4 times I upgrade, the new WordPress managed to break something completely.
I don’t particularly know why it broke. I do know that the nightly backups I keep for just such occasions did their work within about 30 minutes.
I believe I will wait a little longer before updating again.
I always wait for a while unless it’s a security upgrade. For my main sites, anyway. I usually upgrade on one of my test installs to see what’s new.
On a single-site install, 3.1 didn’t break anything for me. I don’t run a multi-site install, so that could well be why.
I’m thinking in future about taking snapshots with git before and after each upgrade for faster rollbacks.
Still, my own fault for being lulled into a false sense of security.