Krugman follows up on his Sunday piece with a recent blog post at NYT and another here where he answers my question.
Obviously the question of whether the college premium is going up, going down or staying flat is a matter of supply and demand for labor. On the demand side, technology can be a substitute for labor, and as tech improves (cheaper, better/smarter machines) the demand for substitutable labor should fall. But labor can also be a complement to technology, so as tech improves we should see an increase in demand. I think the jury is out in terms of the net effect, but clearly this is a matter of what types of labor serve as complements and what types are easily substituted.
Good stuff here from NBER.
As for the supply... here's a bit of info from the economist.
Interesting and related material here.