After many hours of troubleshooting and rebuilding servers into the wee hours of the morning, my coworkers have determined, through trial and error and much frustration, that if you downloaded Exchange 2007 32-bit in December 2006, you will have a working unified messaging install.
If you have downloaded Exchange 2007 32-bit from Microsoft's site since sometime in January or February 2007, you will most likely receive an MSI error code 1603. We've reported it, through many avenues, to Microsoft, so that they will hopefully check and possibly fix the binaries that are available for download from Microsoft's site.
I will edit/revise this entry once I have more solid information but I figured this was worth writing about!