Meditation is, essentially, the art of concentration. It is learning how to control your mindstate, guide your consciousness, and maintain a perception. Lots of (often innexperienced) people insist that it makes no difference whether or not you smoke weed before meditation, that it just makes it "easier". This isn't true. Drugs of many sorts can work as beneficial tools for certain types of mental exploration, but only for their particular advantage. If you want to experience true meditation, and the true deep meditative state that carries with it a clarity like none other, then you have to go in drug free.
Otherwise, weed or anything else will block your perceptions from going past a certain point. They will force you mind into percieving the world in a way normally induced by the substance, and you can not expect to go much further. I'll undoubtedly have many people come into this thread and tell me how wrong I am, how they smoke weed all the time and meditate etc etc. But what I can tell you from my experience of 4 years meditating is that, whether some stoners like to admit it or not, weed and any other drug forces your perceptions. The more experienced with meditation you get, the more you start to see the subtle difference perception makes in your waking and meditative life. And there is a certain type of perception, a pure, enlightened, truly incredible perception, that arises strictly of your mind's own accord.
"Nirvana is the extinction of self" - Buddha