This troubleshooting article
suggests powering down and restarting everything, including the WiFi router. I did all of this. The problem was resolved only temporarily! After two quits and relaunches of iTunes, the Devices menu failed to appear, again.
Apple needs to recognize a problem in iOS 6 and/or iTunes 10.7.
I have had the same issues with my iphone 4s and ipad. I thought it might have something to do with upgrading my older (late 2007) mb pro to Lion. Sounds like you are still on mountain lion and having the same issue so maybe it is something in the ios or itunes software. Odd though as I tried today to sync at home on my older mb pro and my iphone did not show in itunes. Tried it on my office imac (much newer but same os and itunes version) and it showed up! Just wondering if my older mb pro is not fully compatible with lion and itunes 10.7. Maybe time for a new one!