Tiger allows a save as PDF-X option, which will retain the shadows in a Pages document.
[robg adds: This also works for Keynote presentations, and is a much nicer solution than purchasing the full-blown Acrobat package (and the other alternatives, suggested in this hint). To use PDF-X, you do not use File -> Export. Instead, use File -> Print, click the PDF button, and then choose Save as PDF-X from the pop-up menu. I tested both programs, and shadows do indeed work with both Pages and Keynote when viewing files in Reader.]

