The keyboard shortcut to switch to the next window in Excel 2011 is now the rather cumbersome Ctrl+Tab. Similarly, switching to the previous window is Ctrl+Shift+Tab.
Unfortunately, I haven't been able to figure out a way to change Excel's behavior to respect the old shortcut, which is otherwise unused. There's no menu command for switching to the next window, so you can't change it by customizing the keyboard shortcuts within Excel, or within the Keyboard System Preference.
[crarko adds: I haven't tested this one. Command+` is the standard shortcut for cycling windows in Mac applications; I'll guess Microsoft changed this to be more like the Windows method.]

