iPad AirPlay to Apple TV - 802.11n problems
I was poking around in the router's admin interface and examined the wireless
settings.  On this particular router, for each frequency band (2.4GHz
and 5GHz) there is a "Wireless Mode" setting that enables high-speed
operation. It has three settings for the 5GHz network:

Up to 54Mbps
Up to 145Mbps
Up to 300Mbps


The factory default is Up to 300Mbps.  I tested all three settings, 
and the only one that works is the slowest Up to 54Mbps.

It appears that AppleTV's 802.11n implementation does not support 
speeds above 54Mbps.