Wi-Fi syncing and Home Sharing with iPhone 5 not working in iTunes after iOS 8 upgrade
You need to have your Iphone connected to a charger before it will be possible to sync over WiFi

From the iOS 8 User Guide:


  If Wi-Fi syncing is turned on, iPhone syncs when it’s connected to a power source, both iPhone and your computer are on and connected to the same wireless network, and iTunes is open on your computer.


EDIT: Update to iOS 8.0.1 and it is working again without connected to a power source.