Yes Virginia, Louis Pasteur did actually pay a significant role in the use of yeast in wine fermentation. Though most commonly known for his work with milk pasturization, in 1857, Louis Pasteur discovered microscopic cells reproducing in soured milk that he thought to be lactic acid yeasts. They turned out to be bacteria, but the important part of his discovery is that the yeasts were alive. Two years later, Pasteur presented, a "Note on Alcohol Fermentation" -- a paper that demonstrated that yeasts growing and reproducing caused fermentation. Prior to that time, it was thought that alcohol and carbon dioxide were produced through a theory of spontaneous generation.
It was later shown that Pasteur was partially correct and partially wrong, but Pasteur made an amazing discovery about yeast cells: they can live without oxygen. From this he concluded that "fermentation is life without oxygen." This is also not true since we now know that yeasts cause better fermentation in the presence of oxygen but at least Pasteur recognized this possibility before anyone else. Pasteur died before two German brothers discovered that yeasts "per se" aren't actually the cause of fermentation.
For more on the subject, read "The Miracle of Wine Yeast" by Jack Keller. You can also learn more about the current impacts of cultured yeast on wine production at the July 2011 First Crush Winemaking Workshop featuring yeast expert, Andre Austin of CellarWise in Paso Robles, California. The workshop will be held at the First Crush Wine Workshop, located at 2995 Pleasant Road in Paso Robles on July 23, 2011, 2 - 5 p.m. Call (805) 434-2772 for more information or to register, or register online.