My problem is that the "Add-New Theme" under Appearance and "Add New"
under Plugins have both disappeared. All other menus appear to be unaffected (including the "Add New User", that is still there). This puzzling, not to mention annoying.
I thought perhaps something had affected my capabilities. However, I've checked my usermeta table and it seems to be correct.
Likewise there appears to be no attempted hacking.
I'm running no capabilities-oriented plugins and I've also turned off all plugins.
Upgrading from 3.0 to 3.0.1 didn't do anything.