Following up on the 2010 study, Jaeggi and University of Michigan people have run a Single N-back study on 60 or so children.
- Abstract: http://www.pnas.org/content/early/2011/06/03/1103228108.abstract
- PDF: http://www.pnas.org/content/early/2011/06/03/1103228108.full.pdf
The abstract is confident and the mainstream coverage unquestioning of the basic claim. But reading it, the data did not seem very solid at all - I will forbear from describing my reservations exactly; I have been accused of being biased against n-backing, however, and I'd appreciate outside opinions, especially from people with expertise in the area.
(Background: Jaeggi 2011 in my DNB FAQ. Don't read it unless you can't render the above requested opinion, since it includes my criticisms.)