If you’re building a Photoshop file with multiple masked layers and would like to make a new mask from those masks, I found a tip at PhotoshopSupport.com that’s just for you. From the way the tip’s written, however, it sounds like this might only work with CS2 since the term "layer groups" (instead of layer sets) is used. If anyone tries this with another version, please inform the rest of the class.
Basically, all you need to do is group a set of layers that are masked by using the Layer > Group Layers command. Then you can simply hit the Add a Layer Mask button at the bottom of the layers palette to mask the masks.