While working on projects, often I had tussles with erst-while database developers who used to drive the design of the system using Data Models. Most of them came from non-OO background and were apt in ER diagrams. But the porblem is that a Data Model cannot and should not drive the OO model. I came across this excellent article by Scott Ambler discussing this dilemma.
In the above article, the author argues that object schemas can be quite different from physical data schemas. Using two lucid examples he shows how different object schemas can map to the same data schema and how it is possible for a single object schema to correctly map to several data schemas.
There are situations where OO varies significantly from ERD, for example in cases of inheritance and non-persistent classes.
Aslo, OO models can convey more specification on the class diagram in representing certain things like associations than ERD can.
I personally believe that if U are building a OO system, then we should first go for OO Class modelling and then think about resolving the OO-relational database impedance mismatch.