Dampening the cloth would work, but if you really want to get it clean, the iKlear solution works miracles. It's safe to use on almost anything, and they give you at least a year's worth in one bottle.
The MacBook pro is anodized aluminum and actually responds well to car products. For the same price as a small bottle of iKlear, I can but a bottle of maguires Quik Detail and fill ~ four small bottles the size of iKlear, and get the same (if not better) results.