Check this out:
"Gradient transparency doesn't print properly"
http://www.rdpslides.com/pptfaq/FAQ00693.htm

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Shawn Toh (tohlz)
Microsoft Most Valuable Professional (MVP PowerPoint)
Site Updated: Sep 01, 2006
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> In my power point slides, some graphics are behind each other, to be able to
> see both graphics. I made the upper graphic transparent. This works fine in
> the presentation but if I print the slide, the graphic in the back will not
> be printed.