The NL OPS in each of the first 3 months has been: .709-.702-.699. The declines seem as surpising as the low levels. And in June the NL has had some games with the DH. So it might have been lower without the inter-league games. The AL has gone .713-.720-.719.
David Pinto of "Baseball Musings" had a good post a couple of days ago about hitting this year vs. last year. See Halfway Point. He shows how HRs, hits, etc. have fallen.
The graph below shows AL runs per game from 1960-2011
It is pretty clear that this year is pretty low (4.26). The last time the AL was lower than 4.26 was in 1981, when it was 4.07. That was a strike year. Taking that out, we need to go back to 1978, when the average was 4.2.
The graph below shows NL runs per game from 1960-2011
The average this year is 4.1. The last time it was lower was in 1992 when it was 3.88.