Where can I find CRM 3.0 entity diagrams for Accounts and Contacts
etc.?

Where can I find technical/white papers or information about CRM
3.0 security overview?

Some interesting places for you to look is:

http://www.microsoft.com/downloads/details.aspx?familyid=DA8D5048-615...

and

http://yourcrmservername/sdk/list.aspx

I'm not sure if there are technical whitepapers available for the security,
but this is handled in the courses (MOC).

Hope this helps,
--
Ronald Lemmen - MSCRM MVP
Avanade Netherlands
http://www.ronaldlemmen.com/