This is a quite famous question. All of us have seen kids movies where a cat drinks milk from a bowl with the cat's name. And this image is so popular that if you ask a kid what should a cat eat it will answer - milk.
The answer of this question is a little more complex. Some experts say that people should not give their cats milk because the cow milk can make them sick and give them diarrhea. This is caused by the so called lactose intolerance. You can learn more about it from wikipedia's article about milk.
My opinion is on the other side. I think that cow's milk acts the same way for humans and for cats. If I drink too much real cow milk I'll have the same symptoms, but I love to drink only one cup per day. And I give to my cats milk from time to time. I am not sure if this is good or bad, they don't show any signs of illness after drinking it. And some of them really love milk. Of course I am careful with kittens or ill cat but till now I haven't got any problems related to milk.
This is only my opinion, I am not an expert. For health and nutrition problems people should consult mainly their vets.
Edit: I found some old pictures of my cats drinking milk. They are sweet!