I actually do the same thing with my data warehouse, having seperate keys for N/A and Unknown. If there really wasn't a part sold on an order line, then the part is N/A. If I have a customer without a customer type, it is Unknown. The difference is that there should be a customer type, but isn't. There doesn't have to be a part.

However, since CustomerType is an attribute and not a dimension itself, and the source system allows NULLs, I originally had the field as a nullable column. This caused a lot of problems with groupings and filters, most of which were due to the old NULL <> NULL rule. It was then that all the NULLs were replaced with Unknown.