This is a bug in iOS 8 - 8.0.2. If you power off your device then power it back on again, the device immediately shows up in iTunes and wifi sync will work again.
However the fix is temporary. For example if you reboot the Mac that is running iTunes, the device will no longer show up in iTunes and you have to turn the phone / iPad back off / on again to get it to wirelessly sync.
USB sync works fine.. Sometimes deselecting / reselecting the wireless sync option gets it working again. Powering the device off / on is quicker and easier though.
This affects all of my iOS 8 devices, iPhones and iPads. The only device that does not have this problem is an old iPhone 4 that is still at iOS 7. That device has never had any wireless sync issues and it continues to work perfectly fine with iTunes 11.4. It's an iOS 8 bug.