Under Windows, there's a long-standing issue with ATH.EXE and AppleMobileDeviceService.exe intermittently locking up.
»discussions.apple.com/thread/430 ··· tstart=0
Once they lock up, iTunes will seem to run fine, but iTunes won't find your phone.
You can go to an administrator cmd prompt and kill these services:
taskkill /f /im AppleMobileDeviceService.exe
taskkill /f /im ATH.exe
net start "Apple Mobile Device"
Now you can run iTunes and wifi syncing should start working again for a while. The more annoying bit is that when these services hang up, they also hog a CPU core.
It's kind of ridiculous that Apple hasn't fixed this yet.--
AT&T U-Hearse - RIP Unlimited Internet 1995-2011