On the 13″ Air & Pro, power & graphics requirements are still prohibitive if they want to keep thinness, battery life and costs in check.

On the “regular” 15″ Pro, a drop-in BTO option isn’t much feasible, again for battery capacity requirements (the Retina Pro has a 95 Wh battery, vs 77.5 Wh, to achieve similar battery life), and because the screen and lid assembly seems to have a pretty different manufacturing process (having a separate manufacturing line just for an optional component is certainly doable but don’t fit much with Apple’s supply philosophy).

And for what concerns a new iteration of the regular Pro with a default Retina cofiguration, I don’t think it will happen, for marketing reasons.

The new Retina Pro is a much better proposition for Apple to build: no optical drive, less I/O components, soldered RAM, custom SSD… it all means smaller BOM, simpler assembly and no aftermarket upgrades, which in turn means more $$$ for Apple.

I think they will keep the Retina display on that model only, to drag customers towards it, and they will phase out the classic Pro when the market will be ready to accept the lack of optical, ethernet, FW, upgradeable RAM and big mechanical drives on the whole portable line-up.

Based on that, my guess is Apple will make the “Retina Display” an add-on, as the article states. The question is, what will the base model’s resolution be? Are they going to keep that at 1440-by-900 or will now get 1680-by-1050 by default? The latter would be nice.

On the other hand the inclusion of expensive technologies is one of the things that, in the past, has prevented Mac hardware from having been fully cost-competitive.

A good strategic move might be to allow high iPad and iPhone margins to partially subsidize lower Mac margins, adding higher-cost technologies to Mac products while not raising prices. High-res screens might be a good example of such a strategy.

This would give Mac products a difficult-to-compete-with technological advantage against their rivals, in addition to competitive pricing, and help maintain the Mac’s status as a halo product.

When apple introduces pricing for retina display (if it chooses to price separately) that version will be seen in the context of other offerings and that plays a role in customer selection as well (see for instance: http://www.slideshare.net/ragsvasan/macbookair )

If you really are interested in finding it, let me know.
-Rags