I just noticed that in Internet Explorer, the right hand column (starting with the Search box) is not next to the main content, but below it. Has it always been that way? Or just since I upgraded WordPress?
Turns out my stylesheet (based on Sub:Lemon) uses the underscore hack. Not sure why it’s behaving this way. Back to the drawing board.
Update: slightly modified stylesheet; the menu column is now absolutely positioned – we’ll see how this works.