I am having an issue with categories "disappearing" in some blogs running version 3.1.2.
The categories are present, but have "disappeared" in the admin area and when you view the blog, all the posts are filed under "uncategorized".
As fas as my hosting tech support is concerned, everything is working correctly with their server and the database is not corruped, yet some blogs work correctly and others don't.
I experienced a similar issue last month and after a few days of frustrating back-and-forth between hosting tech support and I the issue seemed to resolve itself.
Does anyone have experience of this?