Adam,

I think we are going to have to agree to disagree.  Your example  in no way represents business logic.  That is simply a data rule.  Again, I urge you to check your definition of business logic.  As for Peter's trust issue, of course there are constraints on the table, there is referential integrity built into the design of the DB, but that isn't business logic.  Think of it this way, business logic is decided by the business and is subject to change.  Margin trading isn't decided by company ABC, it is decided by a set of rules that superceede that business.  The logic that decides if trader A is qualified for a margin account is business logic and once that is decided then your data validation rule comes into play. 

It would really help me if you could give me a real world example of something you have done that speaks to this issue. 

Also, I am truely hurt that I wasn't invited to the beer discussion at PASS.