As far as I know, the only way to really insert a 3D object into ppt
and retain it's 3D-ness (i.e. not inserting just a rendered image) is
with a tool called Deep Publish by Right Hemisphere. If you do anything
with 3D you absolutely must check out their products (Deep Explorer and
Deep Publish, especially). They allow you to import from a variety of
formats and inject the model straight into ppt.
Deep Publish is not free. To view the resulting ppt presentation you'll
need to have either Deep Publish itself installed or, alternatively, a
viewer-only component called Deep View. The good news: Deep View is
free. The not-so-good-news: it doesn't work with PPT Viewer, so whoever
wants to look at your presentation will probably need an actual
PowerPoint installed, and an advanced one at that.
A few weeks ago I asked a question in this newsgroup about importing
animation with transparency. Deep Publish lets you do that, too, and a
lot more besides.
(Disclaimer: I'm not in any way affiliated with Right Hemisphere).
Hope that helps,
Alon