What does keybd_event() do?
It seems to send WM_KEY and WM_KEYDOWN events to a window http://msdn.microsoft.com/en-us/library/ms646304%28VS.85%29.aspx
I suspect that Windows might not hooking the keyboard for DirectInput usage, but attaches to some lower layer in the input stack.
I'm not finding the exact paragraph now, but I remember reading something along this lines in MSDN.