Hey ive recently had the same problem. I tried connecting it to a power source and had it on the same network with iTunes open on my PC (which is running windows 7 btw) and it didn't work. I then remember a trick I used for when my safari on my 3gs wasn't working. On your phone go to Settings>Wi-Fi and then press the little blue/white arrow on the far right of your network name, scroll down and then press renew lease. I tried it once and it didn't work, but I then spammed it over over over again and after about 4 or 5 times i tried the Wi-Fi sync again, and it worked!
Hope this helps.
I've been having trouble syncing my 4S to iTunes today. There are many threads which cover this and I've put together this summary of recommendations. Yes, some are quite basic, but bear with me. Let me know if this helps.
Wi-Fi Syncing in iOS 5
1. Have OS 10.6.8 or 10.7 or better on Mac
2. Connect iDevice to iTunes via cable
3. Go to iDevice Summary display/tab: Check "Sync with this iDevice over wifi"
5. Unplug (do not eject) from iTunes
6. Plug into another power source
7. Attempt manual sync (Settings - General - iTunes Wi-Fi Sync - Sync Now)
- iDevice should periodically sync when plugged into a power source
-- if connected to wifi
-- if iTunes is open/running
- Manually starting wifi sync should work even when not connected to power (use caution)
- Heavy traffic on wifi network may disrupt or prevent sync with iTunes
- Using Wi-Fi Sync on Windows is reportedly problematic. It may stop working or work intermittently.
- iTunes 10.5 beta 9 may NOT work with iPhone 4S
- Confirm OS version on Mac
- Confirm using iTunes 10.5 or newer
- Make sure you have already synced iDevice using cable
- In iTunes Preferences - Devices, make sure "Prevent [iDevices] from automatically syncing" is not checked.
- Confirm no firewall (OS or 3rd party) is blocking connection (on Mac, turn off firewall if you cannot connect)
- Restart iOS device
- Restart iTunes
- Restart computer
- Restart wireless router
- Some report clicking "Renew Lease" on network page may help
- After taking one, several, or all of these steps return to step #2 at top.