IPhone and PDA/Blackberry/Palm on a family plan, billing ?'s?

I am looking to get an iPhone and my old lady wants a Blackberry type device(that is not an iPhone). Does anybody know how AT&T/Cingular deals with the billing for this? We want a family plan. Are the iPhone and PDA type phones billed the same? Or would we have to get 2 separete plans, costing a fortune?