Saying that one is agnostic is most definitely an answer to the question "Do You Believe in God?" There's another question that's equally important, which is "Do you believe there is NO God?". Let's examine how three groups of people, namely atheists, theists, and agnostics would respond to these two questions.
Firstly theists - You don't have to be a rocket scientist to figure this one out. Yes I beleive in God, no I don't believe that there isn't a God.
Atheists - No I don't believe in God, yes I do believe there isn't a God.
Agnostics - This is where it gets tricky, because an agnostic is actually answering NO to BOTH questions. Do you believe in God? Well no, because I don't see any avidence that proves that such a being exists. Do you believe there is NO God? Well no, because I see no evidence that leads me to believe that there is certainly NO God in one sense or another.
A more "sitting on the fence" answer that agnostics will give is their answer to the question "Does God exist?" An agnostic will simply say I don't know. I threw this in to illustrate the difference between "Believing in God" and saying firmly that he "Exists". There is certainly a difference here. If I'm not sure that god exists, surely I won't believe in him if I'm coming from a simply rational perspective. But that's where faith comes in for both theists and atheists, in my opinion - If you can prove to me that God either exists or doesn't exist then I will hop camp with you, and commit to being either an atheist or a theist - until then, you've got to have faith that he either does or doesn't exist! I'll be over here on the fence if anyone needs me