Function keystrokes 'ate' by host when running Win7 in a VirtualBox
With 10.6 and a USB keyboard, the Fn keys pass through to the app; OS-X only intercepts the media functions.

In your OS-X's Keyboard SysPrefs pane, is: 


  Use F1, F2, etc. keys as standard
  function keys


selected?

With this selection, my (unshifted) F-keys pass straight through to my VBox VM.  With it unselected, I need to use the Fn key to generate type them (even within OS-X).