I guess my point is that while there is the difference in Nullability, an NC-PK isn't really going to have any I/O advantage over a Unique NCI as the user thinks. The confusion between "PK" and "Clustered" may be exactly what is article-worthy here because a lot of folks think that a PK has to be the Clustered Index.
--Jeff Moden
Change is inevitable... Change for the better is not.