With the reports that the Houston Astros are moving to the American League in 2013, now's probably a good time to take a look at the consequences of the move. A reconfiguration of a division results in more than the Astros simply playing different teams and more frequent interleague play.
In the short-term, a rebuilding Houston moving to the American League actually makes the leagues more comparable in terms of quality. The AL has a .533 winning percentage in interleague play the last three years. Remove the Astros and their 13-32 record against the AL over that period from the equation, and it moves the needle a third of the way to even, leaving the AL at .522 against the National League.
As the rebuilding has gone full-bore, that possible effect increases. If you assume that Houston's .354 winning percentage against the NL would have held up in interleague play against the NL, the AL with Houston would have been down to 127-125 against the NL. That's the closest the two leagues have been since the AL went 127-125 in interleague play in 2004.