Dunno what's going on, but the main menu pulldowns aren't working: if I click on "Search" or "Quick Links", the menu flashes up and then the whole screen flickers and it goes away. And now I'm getting odd flickering whenever I mouse over an edit control in this screen. It almost looks like, when a control/menu/whatever is refreshing, the whole screen redraws instead.
I've tried opening another copy of the site in another tab, same symptoms. And I've got SMC running in another tab, no problem, so it doesn't seem to be a vBulletin issue.