Suspected bug: the .x file importer reads emissive material properties, and interprets them as ambient material properties.
This seems strange, since emissive properties could be mapped to emissive properties, and ambient properties are usually a copy of diffuse material properties!
Even stranger, this seems to have been deliberately introduced between revision 25 and revision 41: http://assimp.svn.sourceforge.net/viewvc/assimp/trunk/code/XFileImporter.cpp?r1=25&r2=41
Is this a bug, or is there a deliberate reason for this behaviour? I've looked around, but couldn't see why this behaviour would be desirable...