solved my iphone 4s sync problem
perhaps someone has already posted this solution, i found this thread while trying to figure out why my iphone 4s stopped showing in iTunes after the ios6 update.
Today I followed a suggestion in another forum and it seems to have fixed the problem.
I turned off "Cellular Data" on my phone, after having done a wired sync. Then I did another sync, with wi-fi sync checked. Still plugged in, I turned cellular data back on on my 4s. Then I disconnected the phone from the sync cable. Voila, iTunes still saw my phone! Closed iTunes and reopened it to test, and phone showed up immediately, and I did a wi-fi sync with no problem.
Hope this helps someone else.