Fevir
Vitaliy I agree with about entity objects, however the question then arises, do you actually have tiers, or do you have what Rocky Lhotka would call "Smart Business Objects" where the objects know how to persist themselves, and therefore, in a purist sense, an absence of a layer?
It's good practice to split application into separate layers (tiers): presentation layer, business logic layer (contains business entities and business flows), application layer (contains application service like tracing, security) and data access layer.
"Smart Business Object" is good idea but not really extensible and maintainable and I don't like it :). From the other side, developing DAL with a lot of DataMapper objects is quite boring. Recently I've developed framework which was able to serialize any Business Entity to database using Business Entity's attributes.