Can I put my old 3G ATT sim card in my new iPhone?

I'm getting a new iPhone 4 from ATT today and they are sending me a new sim with it. Do I have to use a whole new sim card and have to go through the hassle of having a new number or can I use my old ATT 3G sim card I've had forever?

You could have that older SIM cut to size. Ask AT&T about it or you could do it yourself. Many guides online.