For the disbelievers, here's more on the role Lactic acid plays as fuel in our bodies. In short, as we exercise so the lactic levels rise simply because it's a top fuel for our fast-burning energy needs. I can recall all of those hideous ear-prick tests that measured lactate levels. Now I wonder why - what were they thinking?
Check out http://www.cytosport.com/science/lacticacid.html and this nice overview.