Soreness may not be the only indicator of a good workout, but it's not a bad one either I believe.
Lactic acid accumulation is no longer considered a cause of DOMS. Lactic acid disperses fairly rapidly, thus it can't explain pain experienced days after exercise.(1)
It's now generally accepted to be the result of damage to the muscle ultra structure and connective tissue. Eccentric contractions which are known to cause more damage than concentric contractions cause the most severe DOMS which lends to creedence to the micro-tear theory.
The presence of the T cells, and granulocytes Eric mentioned are further indicators of cell damage as these appear because of inflamation brought about by the afore mentioned damage as part of the immune system. This inflamation occurs not only just in the epimysium which surrounds the muscle but throughout the entire muscle.
I'm curious how hypoxia-inducible factor is applicable though. I understand it to be a first responder to allow genetic adaptation to an oxygen-debited cellular ambiance. Likewise, leukaemia inhibitory factor I thought was involved in pregnancy allowing blastocyst implantation.
The resulting cellular inflamation and concomidant imperceptible swelling triggers nerve endings that cause pain. That's why the pain is delayed and why it lasts a few days. The inflamation takes about a day to kick in fully and then a few days to do it's job and abate. That's why anti-inflamatories (which I don't recomend) work so well with this kind of pain.
An interesting new theory supported by some research suggests that the rebuilding process itself by the addition of new reinforcing sarcomeres actually is responsible for the pain. This process causes swelling and puts pressure on nerves.(2)
Thus soreness/DOMS at least seems to be an indicator of muscle damage that we know is at least one precursor for hypertrophy. Obviously from observations we know it's not absolutely neccessary for growth but may be a good indicator when it is present. The other school of thought says that chronic DOMS leads to overtraining and is counterproductive. I personally think the truth lies somewhere in the middle. Though not being necessary I believe it's not a bad thing either..
Oh yeah, BTW, whatever you do, don't ice a sore muscle!!
(1) DOMS at Sports Injury Bulletin
(2) Roth, S. (2006, January 23). Why does lactic acid build up in muscles? And why does it cause soreness? ScientificAmerica.com. Retrieved on July 24, 2006.
Iron