Gothamist took a wine class recently, and in the course of explaining the methods by which one evaluates wine--sight, smell, taste, touch--our teacher told us that there are two theories to explain the tradition of touching two glasses together in a toast. The first, and perhaps more logical explanation, is that it's a way of incorporating the fifth sense--sound--into the wine experience.
The second explanation is historical. When most countries were ruled by monarchies, poison was an oft-employed assassination tool. Nobles would clink their wine glasses together before drinking in order to evaluate the effect it had on the wine. If the wine grew cloudy, it was likely that there was poison in it. If it didn't, it was fine to drink. Less plausible perhaps, but definitely interesting. Cheers!