WiFi Sync Stopped Working

I had been using WiFi sync successfully for some time, with my iPhone 4 and iPad 2 syncing to my iMac. The iPad 2 continues to sync as before, but for some reason the iPhone 4 simply does not appear in iTunes unless I plug it in via USB.

Like I said, both devices had been syncing just fine. When I plug in the iPhone, on the Summary tab the "Sync with this iPhone over Wi-Fi" option is checked.

But as soon as I unplug the iPhone it disappears from iTunes.

The iPhone and iPad are both running iOS 5.0.1 and the iMac is running 10.6.8. I had been running iTunes 10.5.1 when the problem started, and updating to 10.5.2 didn't help.

I have tried rebooting the iPhone, restarting iTunes, unchecking the WiFi sync option, then re-checking it. Nothing seems to help.

Any thoughts about how to solve this?

Thanks,

Jim