Is a Candidate Key a form of Business Rule? Are Referential Integrity rules a form of Business Rules?

At the moment, I am using VFP to design the user services, business services and data services layers of my applications. I want to make sure that my design allows me the greatest possible independence of databases and programming languages. I would like to be able to move the data to Microsoft SQL Server, Oracle or almost any other back-end database. I would like the ability to design a front end using Visual Basic, ASP or almost any other development system.

I would appreciate advice on whether to enforce the uniqueness of key fields or expressions in a 3 (or more) tier structure in the data services or the business services layer. Similarly, should Referential Integrity be maintained in the data services or business services layer?