Steve,
Does this link work for Acrobat 3D version 8.1.3? It looks like it doesn't. If it only works with Version 9, when was this update posted?

Right now we are no happy with Adobe because last year about this time the update for the latest version of Inventor was released which only gave someone a few months before the next version of Inventor was released and it stopped working again. This lack of compatibility is causing us to use other methods than Adobe to communicate our 3d projects.