If you haven't read this, this is a short article on what Apple does so differently than almost all of its competitors - never announce something before it is shippable (e.g. production-qualified and vetted, supply chain and all)

http://counternotions.com/2008/08/12/concept-products/

I bet Apple threw away a dozen such concepts prior to designing the iPad, launching the iPhone (and subsequently iPad).

I remember reading somewhere during the post mortem that they did actually go some ways to start building it.