Got the same issue !
I thought I had a faulty 4S until I saw this thread ...
The airplay from iPhone 4S music player sometimes drops the whole network, sometimes gets the Zeppelin into "Limbo Mode" that needs complete restart.
I experince this drop everywhere between 30 sec - 1 hour of airplay.
I tried resetting then reinstalling each one of them (iOS 5, BW ZeppelinAir and TimeCapsule).
Nothing helped.
Airplay from my MacBook does not stop even after 10 continuous hours.
Airplay from an iPhone 4 works until the battery finishes.
I looked into my ruter logs and I think airplay drops exactly when it rotates the TKIP group key for the WPA security.
My guessing is that there is a bug in the iPhone 4S WiFi software driver.
This could explain why it drops the whole network.
iPhone 4S has a new WiFi network card that supports wireless-n over 2.4 Ghz (65 Mbps).
iPhone 4 has only wireless-g (54 Mbps). So the new WiFi software might have problems.
I might be wrong on this .... but I'm sure there's nothing wrong with the Zeppelin or TimeCapsule.
Cheers !
Equipment I used to test this:
Time Capsule 3rd Gen
B&W Zeppelin Air
iPhone 4S 64GB Black
iPhone 4 16GB Black
MacBook Pro Early 2011