The skeptic blog Watts Up With That chime in with a very brief discussion of Cliff Mass' objections to the paper. Cliff Mass' objections are many - the "lack" of peer review in invited PNAS contributions seems to be a major point of contention (certain PNAS articles are reviewed by author-suggested editors, rather than blind peer reviews), but the major point is the question of attribution. Mass' point is that if AGW contributes 1 degC, then the true contribution of AGW to a 6 degC heatwave is only 1/6 (we would have had a 5 degC anomaly anyway, so the thinking goes). This presupposes a linear climate response and nothing like climate thresholds or non-linear responses, so there are issues with this way of thinking. However, the discussion on all of the sites is instructive - from political, scientific, and rhetorical perspectives - as well as being instructive on the types of arguments that any paper tackling global warming will face in the public sphere.
What's interesting is there isn't a lot of discussion about the general shift of the distribution to the right, and the decrease in -2 sigma events - i.e., more warmer-than-average seasons and fewer cooler-than-average seasons. Nor is there a lot of discussion about overall climate variability, which is something to consider in these types of exchanges.