Bitter experience has taught me ... keep your gui, business and data layers separate.
Always ask - what would happen if I had to access the business logic from a command line, or programmatically (e.g. using COM)? How much would it take to use my component/black box/business logic from a web page rather than from a client-side "thick" gui or console app? What would happen if the database back-end changed from server X to server Y?
I've written too much code when all three layers are all mixed up and it's "impossible" to move to different technologies ...