>Somthing like an account number is fine as a candidate key - something an
>end-user or a customer - say a bank customer who needs to refer to his/her bank
>account number - needs to refer to. Yes, it is a unique number. However, as a
>PK - to use something like an account number as a basis of a relationship
>between database entities - is not correct.

Exactly. To continue the bank metaphor, what happens when the two banks merge? That's when using GUIDs for primary keys starts looking awfully attractive....