Adding salt increases the boiling temperature of water, so it takes a bit longer to get your pot to boil. With questions of food and science, we always turn to Harold McGee first, author of On Food and Cooking. In his section on Boiling Points, he notes that it takes one ounce of salt per quart of water "to raise the boiling point a negligible 1°F." So we're not talking much here.
Most people believe in salting the water just as it comes to a boil, or right after. So how much salt do you add? At my house, we add a little less than a tablespoon for every pound of pasta we cook. I'm afraid to say that this isn't based on science, but merely on taste. As the folks at Fine Cooking note, "A generous amount of salt in the water seasons the pasta internally as it absorbs liquid and swells. The pasta dish may even require less salt overall." So we're leaning a little more towards the flavor side of things and farther away from the science on this one. If an ounce of salt only raises the boiling point of water 1°F, you'd need a whole lot of salt to make a noticeable difference cooking-wise. We lay this one to rest with a conclusion that salted pasta water leads to tastier pasta. And tastier pasta makes everyone happy. Case closed.