After 252 interleague games, we have a final tally for 2012: AL 142, NL 110. That's a .563 winning percentage for the AL, which translates to a 91-win pace over a 162-game season. The AL has now taken the NL's lunch money in interleague play for nine consecutive seasons.
Here's what the AL's yearly winning percentage has looked like since the start of interleague play:
Overall, the AL has won at a .525 clip since 1997, going 2079-1883. Since the winning streak started in 2004, the AL's record is 1246-1020 (.550). The NL actually narrowed the gap for three consecutive seasons from 2009-11, but the AL lengthened its lead this year, possibly as a result of importing more talent over the offseason.
Several reasons have been proposed for the AL's recent run of success—this summary by Justin Inaz is a couple years old, but it still pretty much covers it. Regardless of the factors responsible for the 21st-century swing toward the AL, league strength is cyclical, and neither league is inherently superior, at least to the extent that the AL has been lately. The NL outclassed the AL over the first seven seasons of interleague, and it will again. We're just not sure when.*
*Though dumping the Astros on the AL next year won't hurt.