When I try to sync my iPhone or iPad via Wi-Fi it happens very often that the symbol of the device disappears after iTunes is telling me that it's searching the device. After a couple of minutes the symbol re-appears. Syncing via USB works flawlessly.
This is very annoying, especially because I don't get any error message. The device's symbol simply disappears.
Best Answer
I have experienced similar issues from time to time. Normally, if the device is ineligible to sync (for example, if it somehow disconnected from the same wifi that the computer is on), it will disappear from iTunes in the same way if you were to disconnect an iOS 4 device that had been plugged in with USB. I'm guessing the device's connection is somewhat faulty, and is ultimately responsible for it's disappearing (this is just my guess/thought).
I have found, however, that if I initiate the sync from the iOS device after it has disappeared, that it will typically come back and work well. This can be done from Settings > General > iTunes Wi-fi Sync.