From Ars Technica:

For instance, users that have jailbroken iPhones would like to have a modified version of iTunes that does not ask to update iOS when the device is plugged in, since the update might overwrite jailbreak modifications," Freeman said.

Try these:

• iTunes > Preferences > General > Check for new software updates automatically — OFF
• iTunes > Preferences > Devices > Prevent iPods, iPhones, and iPads from syncing automatically — ON

No system hack necessary.

Another thought: We don’t normally include the serial comma in British English (the one before the ‘and’ in the second path above). Should localisation take conventions such as this into account?