I was hoping for a more definitive answer -> has anyone converted an office
openxml file to an earlier version?
Well yes. In PPT 2007, File, Save As and choose the earlier file format.
Are the results perfect? No. It's still in beta, for one, and for another, the
earlier versions don't support all the graphics features of PPT 2007. The conversion
will never be perfect.
Have you downloaded the beta preview and tried it? It seems that would allow you to
answer your specific questions re the specific features you use in your own
presentations.