Another recent article on group selection:

Henrich et al (2010). Markets, religion, community size, and the evolution of fairness and punishment. Science 327, March 19 2010, p. 1480.

Their theory: "Larger and more-complex societies prospered and spread to the degree that their norms and institutions effectively sustained successful interaction in ever-widening socioeconomic spheres, well beyond individuals' local networks of kin and long-term relationships."

Their evidence: "Using 3 behavioral experiments administered across 15 diverse populations, we show that market integration (measured as percentage of purchased calories) positively covaries with fairness while community size positively covaries with punishment."