I just had to post this.
When iOS 5 came out a while back (months ago), I had tried to use WiFi syncing. I never succeeded.
For some reason, when my iPhone (4S) was plugged into the wall, and both the iPhone were connected on my only wireless network, the phone would never show up in the iTunes device list. Also, on the iPhone, when I went to Settings / Wifi syncing, the Synchronize button was greyed out.
What was even weirder was that I could see my iPhone's music as a shared library on iTunes on my Mac, AND, I was also using my iPhone Remote app to control iTunes on the Mac remotely.
But, WiFi syncing wasn't working.
I read many troubleshooting threads either on the Apple official forum or elsewhere. Nothing worked, so I left it at that.
Today, I must have spent another 2 hours on the problem, same result.
That is, until I changed one of the variables : the network.
Instead of trying to connect both the Macbook and the iPhone through my wireless router, what I did was to create an Ad-Hoc network on the Macbook, and tried to make WiFi syncing work through that network.
IT WORKED !
If you want to try it :
Apple menu / Preferences / Network, then click on the list-box showing your current WiFi network (right section), and select "Create a network". Name that network "Test Network", and click OK.
Now, on your iPhone, go to the Settings app and select that new network for WiFi.
I may have restarted iTunes, or maybe not, but the iPhone showed up in the Devices section, and the WiFi sync kicked in.
I then reconnected the Macbook and iPhone to my original WiFi network, and the iPhone still showed up in the Devices section.
It's like using that other network cleaned something up in the Macbook's or iPhone's internals or something.
Hope it helps someone.