Turns out the reason is a conflict with the shortcut Alt+Shift in the keyboard layout switcher (System > Preferences > Keyboard > Layouts > Other Options... > Layout Switching > Alt + Shift change layout.) Changing the layout shortcut key allows Shift-Alt-Tab to work for the application switcher.
What must be fixed to make both shortcut combinations work together? It works fine in Windows XP. ;) xev shows Alt-Shift as ISO_Prev, if that helps.
Turns out the reason is a conflict with the shortcut Alt+Shift in the keyboard layout switcher (System > Preferences > Keyboard > Layouts > Other Options... > Layout Switching > Alt + Shift change layout.) Changing the layout shortcut key allows Shift-Alt-Tab to work for the application switcher.
What must be fixed to make both shortcut combinations work together? It works fine in Windows XP. ;) xev shows Alt-Shift as ISO_Prev, if that helps.