I just created a new world in the latest snapshot and as I loaded in, I noticed that the leaf blocks were no longer transparent between leaves and instead were just black. I tried reloading the world and my game but to the same results. I'm not using a texture pack. This only affects leaves and not other foliage in the area (grass, vines, bamboo)
What is your graphics quality set to? Fast graphics do this to help performance, whereas Fancy and Fabulous settings have transparent leaves. It would explain this if you were on Fast graphics.