I think with continued advancements in base oil technology we are going to see "thinner" oils becoming acceptable in applications where "thicker" oils were/are the norm.
The ONLY reason Ford could make the switch to 5w20 3 years ago was because the quality of "conventinal" base oils had progressed to the point that this grade could not only be made, but could also protect and last in a modern engine. Before the advent of all-hydroprocessed base oils (Group II, II+, III), it would have been nearly impossible to produce a non-synthetic 5w20 that would go the distance in a modern engine.
Remember when Mobil 1 was first introduced in the US? It was a 5w20 (the only grade it was made in). Mobil advertised it as being able to protect as well as a conventional 10w40. They also advertised it as beeing good for 1 year or 25,000 miles. Needless to say, it didn't sell too well, and it being a 5w20 was a big part of the reason. But the fact remains that the reason they could produce a 5w20 and seriously recommend its use at at a time when 10w40 was the norm was because of the the superior base oil being used. The same holds true now for the 5w20s being sold that meet Ford's stringent specs.