I've thought about this as well. Since handguns have pretty much replaced swords and the like as personal weapons, it seems that a handgun-based martial art would be cool and maybe even useful.
Another movie to check out is Ghost Dog: Way of the Samurai. It's about a hitman who follows the samurai bushido code and stuff. It's been a while since I've seen it, but I remember that he uses handguns in a prety graceful, martial-arts type of way. He used two handguns, but not in the normal, both blasting in the same direction style, which I found interesting.