As mentioned above, this problem has somehow been eradicated for me. I'm not sure exactly what fixed the issue for me, but I can say I did the following things - in this exact order - after which the issue was no more:
1) Installed the latest version of iTunes (11.1).
2) Ran a number of system maintenance tasks, for which I use the application "OnyX". In particular, I verified "S.M.A.R.T status" and "structure of volume", then verify/repair "System Permissions", then ran daily, weekly, and monthly "Scripts", then ran a rebuild of "LaunchServices", then ran a number of "Cleaning" tasks (including all "System, User, and Misc." tasks).
3) Restarted my iPhone, my iPad, and the computer used to sync these devices (my iMac).
I'm not sure if one (or any) of these things rectified the problem for me, but I'm sure delighted it's been rectified.
Hope this helps.