Is there a second-generation Airport Express 802.11n?
Not yet on a 2nd gen 802.11n Express.

Apple has published four different manuals on the Airport Express:


AirPort Express 802.11n (1st Generation) - Setup Guide


The three expresses prior to that were not 802.11n so perhaps that's where the three previous generations (and the 1st and only newer generation names have come to be) A tad confusing, no?