Opacity masks are a feature in Adobe Illustrator which helps to bridge the gap between bitmapped artwork and vector artwork. Opacity masks allow for smooth, gradient-like transitions between colors, something that is not possible with straight bezier-drawn vector points in an illustration.
Care should be given about what version of Illustrator your printer has. If they have very old legacy versions (earlier than version 7.0), the masks are not going to work. (Sidenote: Version 8.0 came out in 1998...it's probably at least time to upgrade to 10.0 for the layer transparency effects). If you are working in Illustrator CS2 and you are sure that you need to save to a legacy version of Illustrator, you will need to flatten the opacity effect. This creates some strange-looking images, and unfortunately some bitmapped art is integrated into the artwork to create the smooth gradient transition.
Below is an image which shows how to make in opacity mask in Adobe Illustrator. First, create an object with a linear gradient on top of the object that you want to mask in the logo design. Then, select both objects. Go to the Transparancy Menu, choose the top right menu and select "Make Opacity Mask"
Below is the result of our efforts
