We're getting ready to offer a business debit card:

Does the Opt In rule apply? Or is that a direct correlation of whether or not you choose to apply your Reg E procedures to the business card program?

Reg E does not cover consumer accounts so you would not need to solicit opt-ins from your business debit card customers if you offer overdraft privledges to you business accounts.

I would advise against applying Reg E procedures to business cardholders as you place the bank in a greater liability position.
Business debit cards will still be covered by VISA/MC's Zero Liability protection and you can utilize similar chargeback procedures to your personal cards, but VISA/MC chargeback procedures are not bound by the investigation timeframes that Reg E requires.