Funny thing is the rMBP's shipping estimates has actually dropped to 1-2 weeks only today! It was 3-4 weeks before: http://www.macrumors.com/2012/07/21/retina-macbook-pro-shipp...

True though don't didn't they initiate the trend in Just In Time manufacturing. Not sure to what scale they do these days having previous read about them buying up mountins of flash memory few years back. I dare suspect that customised models like the macbook pro which are built to your spec from the choices you get a more accomodating item for building to order. Few standard configs will be in the shops and if you want something different they can get it for you. Items ike iPads they can bulk build as little variance/permutations as its just how much memory in them for storage. But yet there has been many a time they have sold out. No supplier realy wants that, but also at the same time it does help with free news and in a way help promote your product as it does have your advantage as long as those who miss out initialy can wait and not purchase a alternative in frustration.

A Dell takes 1-2 weeks to ship too, possibly more...

It's not a matter of demand. It's a matter of acquiring the components, building the product to spec, testing it, and preparing it for shipment.