Jose, yes, there existed an ERD some time around. I guess it stretched over ton's of pages and was not useful at all.
As for the changes, they are numerous, but the basics should still be fine. What is more important though for the evaluation as a Toolki is not the datamodell but the API that is provided to access the data modell (in my opinion).
I you want to create an ERD of the core functionality, install OpenACS in PostgreSQL and use http://www.rbt.ca/autodoc/ PostgreSQL autodoc to create a DIA script that you can use for the ERD. I have not tried this way, but I think Joel did it once.
