Jdeveloper 10g finally has a data modelling diagram, which will allow you to create ER models for existing tables in your database. You can also use to it to design a new ER model, and let jdeveloper create a ddl script to create the objects in the database. Jdeveloper can also create an alter script which will update an existing schema with the changes you made in the data modelling diagram. There are still some minor problems with this. For example, if you change an attribute constraint from not null to nullable, this isn’t detected as a change by jdeveloper.
When you want to create an ER diagram for an existing schema you can simple drag the tables from your database connection onto a data modelling diagram. Jdeveloper will also draw the foreign key relationships, but only if you drag the tables involved in this relationship, at the sametime onto the diagram. This doesn’t seem to work if you drag the tables one by one.