I've been bitten by this too. I love OmniGraffle and would prefer to build my PowerPoint shows using it. I mean... the alternative is the built-in uSoft box-editor-thing. Yuck.

But because OmniGraffle copy always grabs a tiff, the resulting PowerPoint show won't work on Windows (even if you install all the QuickTime components).

Anyway, in pinch, the quickest thing to do was export the PowerPoint show as a QuickTime movie. That played ok on the Windows PC.

But my feature request for the future: A preference to set the image-type for copy operations. If it'd been gif or jpeg or png, it would have just worked.