Ever since Koen talked about the centrality of reproducibility to design, as a distinction between design and architecture, I have been contemplating the term "interior design" and finding it problematic and wanting as a referent.
What one does to the interior of a building would most fittingly be called either: a) interior architecture, if you alter the space program and surfaces; or b) interior decoration, if you focus on furniture and ornamentation.
I suppose one could be an interior designer of the interiors of mobile homes and prefabbed houses that are duplicated repeatedly in production, or even for homes in subdivisions where the home is repeated multiple times on various lots in the subdivision. But for new houses that are customs, or for existing houses with interiors that are being rehabbed, whether originally custom, or duplicated in large numbers, it would seem that one would be an interior architect, or an interior decorator.
I know. Its only semantics in one sense. But I think the vagaries in naming may betray a bit of vagary at the heart of what the discipline and field are all about.