This. Feffer has said that the season ticket base is the biggest since 2008 (first year at park).
If one takes a look at the lowest-ever announced attendances in each of Nats Park's five seasons, there may be some substantiation for that:
2008 - 20,657 (Tuesday 23rd September vs Marlins)
2009 - 12,464 (Monday 20th April vs Braves)
2010 - 10,999 (Monday 20th September vs Astros)
2011 - 13,413 (Tuesday 12th April vs Phillies [!])
2012 to date - 14,520 (Wednesday 18th April vs Astros)
Average announced attendance this season is 25,332/fixture, up 1.8% from last year's full-season average of 24,877 (and this latter number includes all of the bigger-drawing summer fixtures, making this year's numbers look even more promising).