[Duugu]

I'm not sure. Maybe the middle button is not registered.

I don't have a 3 button mouse available atm and can't test it.
Try to register for MiddleButtonDown:

for container = 1, 5 do
        for item = 1, 36 do
            _G["ContainerFrame"..container.."Item"..item]:RegisterForClicks("MiddleButtonDown")
            _G["ContainerFrame"..container.."Item"..item]:HookScript("OnClick", function(self, button)
                print(button)
                if IsShiftKeyDown() and button == "MiddleButton" then
                    print(GetItemInfo(GetContainerItemID(self:GetParent():GetID(), self:GetID())))
                end
            end)
        end
    end

See http://www.wowwiki.com/API_Button_RegisterForClicks for reference.