The other day my iPhone stopped showing up in iTunes. I followed these instructions to fix it. Here are the simple steps I took:

1. Disconnect iPhone.
2. In Terminal.app type:

   rm /System/Library/Extensions/AppleMobileDevice.kext

3. Then:

   rm /Library/Receipts/AppleMobileDeviceSupport.pkg

4. Restart.
5. Download and install current version of iTunes.
6. Connect iPhone again.
7. Open iTunes.

Voila! iPhone showed back up. This took me a bit to figure out, so hopefully it saves you some time.