>Agreed. But what is a business rule vs. a RI rule. One client may say that he
>needs to have a customer for every invoice that is entered and another may say
>that they need to be able to enter an invoice and associate it with a client
>later(idea from 'About Face').

I don't remember which MVP I stole this example from (was it you, Jim?), but say you have a numeric field to store the day of the week. A Database Rule is that the values must be between 1 and 7 (or 0 and 6, depending on how you count it). A Business Rule is that you are never open on weekends. If you put that in the database, and then have a special sale on Saturday, you have no way to enter that, because the Database won't let you.