Although Web transparency allows some fluidity with design elements, it can also be used to create some very advanced Web effects, such as sprite movements or passing a shadow over background elements. (See an example at http://members.aol.com/thebookdoc/articles/Balloon.html.) More advanced printing effects include making high-resolution (2540 dpi) bitmap drop shadows that can be arranged as part of layout. These drop shadows act almost exactly like the balloon shadow from the Web example, but at a higher resolution. Essentially, the white pixels are ignored when printing the bitmap, and the image will let whatever is underneath peek through.
If you aren't clear about any of the techniques or ideas described in this article, you can find out more in Special Edition Using Adobe Photoshop 6, as follows:
Chapter 4: "Working with Channels"
Chapter 5: "Selection and Masking"
Chapter 14: "Choosing an Image Mode"
Chapter 15: "Selecting a File Format"
Chapter 16: "Understanding Resolution"
Section X: "Creating and Using Web Graphics
Section XI: "Image Printing Output and Use"