It has been a while since I've posted anything to this thread, but I have been following it. I thought I'd reply to your message.
You seem to be experiencing the same symptoms I am having.
The following is 100% reproducable for me:
1. Reboot my Mac and open iTunes.
2. My iOS devices all appear ready for Wi-Fi sync, and syncing works fine at this point.
3. Put the Mac to sleep (wait until the sleep procedure is completed (usually fans stop spinning)).
4. Then wake the Mac from sleep.
5. Open iTunes again and no iOS devices will appear (unless they are attached via USB).
6. After this point, iOS devices will never appear for Wi-Fi sync until the next time I reboot the Mac.
I am running Mavericks and have an iOS 7 and iOS 6 device - the problem affects both of them.
The majority of the cases I read on this thread of people successfully getting Wi-Fi sync working seem to include a reboot of their Mac at some point. This is the key step... It will continue to sync successfully via Wi-Fi until the Mac goes to sleep.
In a previous message in this thread from me and from another user (sorry, don't remember the name) it was also discovered that if you open the Console utility, you can see some suspicious messages about a device being ignored because the machine thinks it's a duplicate (something like that).
I have reported this information as a Bug Report to Apple.
There is an NDA related to any bug reports, but I think I'm safe to simply say that I have reported it as a bug.