In my games, there's definitely an absolute, clear-cut good and an absolute, clear-cut evil. There's plenty of room for shades of gray, but good and evil aren't just ideas in a D&D setting--they are forces that strive against one another and shape the cosmos.
In previous versions of the game, characters could literally detect the presence of good or evil creatures, magic, artifacts and the like with spells. You can also detect traps, treasure, secret doors, certain creatures and the like with spells. On this basis, I posit that good and evil exist in some literal, concrete form in the assumed D&D setting, at least in 3.5 and previous versions.
One of the things I actually don't like about 4E is the move away from implied moral absolutes. Radiant and necrotic damage have replaced effects that would have affected creatures based on their alignment. To me, this has removed flavor and mystique from divine characters. Instead of being warriors empowered to smite the wicked, they are warriors that are good at dealing radiant damage. As a DM, it's easy enough for me to refluff this to my taste, but I still prefer the idea that characters are fighting against a grand cosmic force.
All that being said, I also think that it's an issue for individual DMs and their groups to decide for themselves. I personally think that you can have moral and ethical absolutes without coming across as didactic. I also think that shades of gray are far more interesting when used with strong doses of black and white for contrast. For example, a clear-cut villain could become sympathetic, or a trusted ally doubted. Conversely, characters might think that someone harboring a complex agenda is really much simpler than he seems to be. Either way, you can make an interesting story by mixing these perspectives. I think it's a false dichotomy to say that you must have either a black-and-white morality or only shades of gray.
It's also incumbent upon the DM to determine moral absolutes for his game. I think that this is why many players prefer shades of gray--they don't agree with a particular DM, and would rather have the narrative license to do as they please rather than frame their actions in a manner consistent with the ethics and morals of the milieu. I believe that the alignment system is purposefully vague to allow individual DMs the leeway to frame situations as they see fit and arbitrate character actions accordingly.