Foreign Keys to tables where primary key is not clustered index
Taken from the MSDN article on FOREIGN KEY Constraints


  A FOREIGN KEY constraint does not have to be linked only to a PRIMARY
  KEY constraint in another table; it can also be defined to reference
  the columns of a UNIQUE constraint in another table.


I think it is perfectly fine to define a UNIQUE CLUSTERED INDEX on an IDENTITY column and define the PRIMARY KEY on different columns, often with a NONCLUSTERED INDEX but you don't have to specify an index.