The Pioneer VSX-921 is a great networked receiver, but AirPlay was very unstable for me: after about 20 minutes streaming was often interrupted, with the Pioneer disappearing from the network.
First I suspected the receiver, then, after talking to a friend who has never experienced these problems, my wireless router. Disabling 802.11N, I was able to stream for 40-50 minutes uninterrupted (the previous record was 30). Then I ditched the router (Belkin N) completely, replaced it with a Billion 6200NXL, and the problems were gone.
This story has no conclusion. AirPlay is new, firmware implementations are immature, and uninterrupted high-bandwidth streaming is a sure way to trigger bugs and corner cases.