I had this same issue in Beta 5. Any attempt to sync both my iPad and iPhone, whether plugged in or wirelessly caused it to tell me that iTunes had been synced with another iOS device and that I needed to either restore from an older backup or set up my device as a new one. I searched around a little bit and found a solution that worked.
Connect your device to your computer with iTunes open. When it shows the screen that asks if you want to restore or set it up as a new device, select new device and click Next. On the next screen, it'll ask you to name your phone. Just click Next again. As soon as iTunes starts the sync session, unplug your device immediately. After a few seconds, iTunes will say that the sync session failed, but the next time you connect your phone, it should work normally again. I didn't have to reset my phone, but it did reset some of my preferences and tried to install all the apps I had in iTunes the next time I did a sync. However, it's much easier to clean that up than to go through a whole backup and restore.
I'll keep this in mind, thanks! I'm doing a full restore right now from a backup.