orbitals
As somebody working in philosophy of science and more specifically philosophy of chemistry I have found the recent discussion very interesting.
Although I may not have fully digested every subtle nuance and technicality concerning orbitals that has been voiced in recent days I believe I may still be able to contribute to one major aspect of the discussion. 
 This is on the question of usefulness and reality.
There is a long-standing debate in philosophy of science going back for hundreds of years which in modern terminology is referred to as the realism versus anti-realism debate.
Very briefly realists believe that we can attribute physical reality to scientific terms that are used in successful scientific theories regardless of whether or not these terms can be observed. Indeed most realists believe that we should attribute physical reality to unobservable terms or entities like quarks.
Modern realism arose as a reaction against logical positivism whereby every scientific term would need to be grounded in observation. The failure of the program of logical positivism (Carnap, Schlick, Nagel, Reichenbach etc.) came about for many reasons among them being the inability to draw a sharp distinction between theoretical and observational terms.
But logical positivism has not died away altogether and there is a sense in which it survives in the modern anti-realism view of scientific theories. According to this view, of which Bas van Fraassen is the leading proponent, observability remains an important criterion. If a scientific term or entity cannot or has not been observed then we have no warrant for assigning physical reality to it. No matter how useful a scientific theory or scientific term might be this does not warrant us in the further belief that this theory or particular scientific term might have physical reality. As I mentioned in an earlier posting there have been many terms that were temporarily useful in now defunct theories. For example phlogiston was initially quite useful and successful in explaining many chemical facts. Some people may even have believed that this meant that phlogiston was a real physical entity and yet as everybody knows it turned out to be a mistake.
Of course I am not suggesting that orbitals will necessarily turn out be be a mistake, except in the general sense that all scientific theories and approaches are eventually refuted. This view, called the pessimistic meta-induction, provides a rather powerful argument against regarding any scientific theory realistically and similarly any scientific terms from current theories.
Finally, it should also be recalled that the most widely held interpretation of quantum mechanics, the Copenhagen interpretation, is essentially an anti-realistic interpretation regardless of what people like Einstein and David Bohm may have wanted. So it is a little puzzling to discover that some quantum chemists are prepared to go beyond this general philosophical framework when it comes to one particular term within quantum chemistry and to attribute physical reality to orbitals. 
 regards,
 eric scerri
 --
 Dr. Eric Scerri ,
 UCLA,
 Department of Chemistry & Biochemistry,
 607 Charles E. Young Drive East,
 Los Angeles,  CA 90095-1569
 USA
 E-mail :   scerri_at_chem.ucla.edu
 tel:  310 206 7443
 fax:  310 206 2061
 Web Page:    http://www.chem.ucla.edu/dept/Faculty/scerri/index.html
 Editor  of  Foundations of Chemistry
 http://www.kluweronline.com/issn/1386-4238
 Also see International Society for the Philosophy of Chemistry
 http://www.georgetown.edu/earleyj/ISPC.html