I just wanted to say that I don't at all agree with the suggestion that this fundamental question is rarely discussed.  On the contrary, I think it's pretty much *all* that is ever discussed in philosophical literature since Descartes.  Nearly every book or article (except those on ethics or aesthetics) that may not seem to be about whether the world is in our head or not--say it's on the nature of number or the question of stimulus synonymy--is generally an attempt to clear up some issue thought to be preliminary to this in-or-out-of-head business or to get at that matter through a side door.  
Are there universals?  Are persons bundles of experiences?  Are there natural kinds?  All are doing their little part in trying to answer this "ignored" question.
W