"Business rules" are rules that aren't required for any technical reason, but are needed to fit in with how your buisness operates. For instance, as far as SQL is concerned, it makes perfect sense to have an order placed without a customer, but that doesn't make any sense to a retail store that runs this database.

The original "developer" implemented the business rules in PL/SQL and triggers. Is this a bad thing?

It depends (:

What languages should the middle-ware be written in?

A turing-complete one would be good (: Again, it all depends.

----
I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident.
-- Schemer

Note: All code is untested, unless otherwise stated