>BTW, I do not know how to handle CASCADE updates/deletes if the FK is Null. What do you think?

Depends. I can see a scenario where you have an optional link to another table - say, a link to addresses, and for some records you have no address at all, so the fk into addresses would be null. In such a case, do nothing. Now if a record is edited so that it had a fk and now that FK is going null (just discovered that the address doesn't exist so you clean up), it's a matter of your policy whether you want the address record to still exist (maybe you have someone else at the same address) or you want to kill it right away (because your table has 1:1 relation with addresses).

Master-detail relationship would be a different matter - no child record should ever be written without a proper key to its parent record.

Don't know whether I said anything useful here - I'm doing RI through business objects for quite a while, or have a DBA who writes the stuff at the SQL server level, so I'm not much of a help when it comes to RI within a dbc.