Just as info for those who may run into this.
I had a customer complaint that ArcMap does not recognize the coordinate system from a GeoTiff file I created with FME.
Well after lots of going here and there, trying out new GeoTiff writer options and the like the solution was very simple.
ArcGIS 9.3.1 does not recognize the coordinate system in the GeoTiff
ArcGIS 10.0 can read and use it without any problem.
Michael