This is a topic that's been discussed in the pages of New Scientist magazine (although this was on the wider subject of not washing at all).
The key point is the word "clean". As you say, the skin exudes sebum - which is itself a cleansing agent. It is the build up of sebum which gives the hair its' greasy appearance. Several scientific studies have shown that sebum builds up as a result of constant use of chemical abrasives and that this continues for some time after the cessation of use of these abrasives. In short, stop using shampoo and in the short term you'll get greasy hair.
Numerous non-controlled tests (ie people who just did it) have shown that when the body and hair are not subjected to constant bombardment by chemical substances, a natural equilibrium is reached. This state would not be defined as "clean" by L'oreal standards but is generally accepted as non-offensive by independent observers.
Put in basic terms - explorers, peace campers, and various other active but non-delinquent groups who for practical reasons have eschewed the standard hygiene diet of western civilised man report that when they encounter Gillettes (the term generally applied to the well-groomed westerner) the results are reported that the smells, and general appearance, are different but non-offensive.
So the short answer is - you won't get Head n Shoulders hair by leaving it to nature; but you will get socially acceptable hair if you leave it long enough to go through the over-reaction stage.
There is one important caveat - there has to be sufficient airflow for the effect. If you cover your head you will need to constantly wash your hair. This also applies to your body.