You can create a clustered index on a column other than primary key column if a nonclustered primary key constraint was specified. http://msdn.microsoft.com/en-us/library/ms186342.aspx
So the above told me: I can create a clustered index on columns other than primary key.
I think it also conveys that a primary key should either be a nonclustered primary key or clustered key. Is it possible a primary key is not indexed?
What's more:
When you create a UNIQUE constraint, a unique nonclustered index is created to enforce a UNIQUE constraint by default. You can specify a unique clustered index if a clustered index on the table does not already exist.
Does this mean a unique constraint has to create a clustered index or a nonclustered index?