The World Imagery map is typically updated twice per year with a large volume of updated imagery. As you mention, the imagery map does include a large amount of NAIP imagery. With each update, we try to include the latest available NAIP imagery that has been processed and blended into the nationwide mosaic.
The next imagery update has been completed and being replicated for deployment to ArcGIS Online in late Q2. It includes a number of updated areas of NAIP 2009 imagery. In the following update scheduled for late this year, there will be any remaining NAIP 2009 imagery plus initial sets of NAIP 2010 imagery, along with other imagery from commercial sources and local providers.
Deane K.
ArcGIS Online Team