Since a couple of days, my Airplay button doesn't work any more. I'm working on OS 10.7.1 Lion with iTunes 10.4.1. Weird thing is my iPhone 4 and iPad 1 still work with airplay. I configured my Airport Express (version 6.3) as a WDS on a 802.1b/g/n network.
It used to work fine. And suddenly my airplay button disappeared. After googeling a bit I found some threads which pointed out the option of enabling the option “Look for remote speakers connected with AirPlay.” in the iTunes preferences. No such thing there for me.