I just attended a lecture this past weekend about muscle and nutrition by Dr. Len Kravitz, PHD. Though everyone says this (myself included), you cannot lose weight and build muscle. You can lose fat and build muscle, but if you are building muscle you will gain weight. In most instances the muscle gain/fat loss will equal out on the scale. Like GQjock says, don't let the scale be your guide. Check how your clothes fit, or use a tape measure. In my experience, most guys show fat loss in their face and neck first and belly last. So if someone asks you if you've lost weight, they've noticed a difference.
Hope this helps to clarify.