I was having this problem too. Go to this link and check each one of the plug-ins that your currently using and make sure you either deactivate it or delete it all together, that is if there’s no update to the plug-in for WordPress 2.3 ;-).
The reason why this is happening is because WordPress 2.3 uses a new implementation of category tables and the like. Basically, your plug-in is calling on a section of code that has been updated or erased, in this case the database.
This took me a couple of minutes of frustration, too, so don’t feel stupid ??