This article explains that alcohol actually enhances the flavor of foods in two ways. First, it helps carry aromas from the plate to our noses, which plays a larger role in our experience of "tasting" food than we might think.
Secondly, alcohol bridges the gap between fat and water molecules. This means that alcohol helps us taste and smell things we wouldn't otherwise be able to perceive quite as well. In a marinades, brines, and bastes, alcohol is also able to penetrate meat better than other liquids, and it carries flavor into the meat itself. The meat winds up tasting much more rich and seasoned than it would otherwise.
One of the other surprising bits of information we picked up from this article was how little alcohol it takes to do all of this. The authors say just a tablespoon of vodka in a marinade or a shot of port in a braise will make a definite difference in flavor.
• Read the Article! - Alcohol's Role in Cooking by David Joachim and Andrew Schloss in Fine Cooking, Apr/May 2010