iTunes/iPhone syncing is done on a "per Mac" basis. In iTunes Preferences, go to Devices, and check the box for "Prevent iPods, iPhones, and iPads from syncing automatically". This will prevent iTunes from opening when a device is plugged in.
UPDATE: There used to be in previous versions of iTunes, in the old "Syncing" Preferences, a way to disable automatic syncing and that prevented iTunes from even opening. But it seems they have changed the behavior because searching for answers on other sites yield the same behavior but in previous versions of iTunes. I set this preference ages ago, so the preference must be holding over.
In response to your comments:
iTunes still starts, still does some
kind of mini-sync, but doesn't
recognise the name of the phone or
allow access to its contents.
Are you sure you are syncing with the iTunes install you originally started with? Syncing is strictly a one-to-one relationship for iPhones and iTunes. Once you sync an iPhone to an iTunes install and wish to sync with a different one (like on another Mac), you are then forced to clear the phone and start anew.
My iTunes behaves the way you are describing, but when I plug in my iPhone, I get the "mini-sync" you mention because that's the device registering with the OS and iTunes that it exists and iTunes puling needed info to interact with it. Can't really get around that one.
it would prevent the machine being
used at all for syncing, no?
No. You could still sync if you wanted, but that's if you are syncing with the iTunes install that recognizes the phone. Otherwise, see my comment above.
BTW, save the downvoting for the really egregious answers. Downvoting costs points. Otherwise, just leave it alone.
This could be a memory issue that the developer was forced to handle this way. All apps in iOS are subject to memory warnings at which point the app (developer) is to free up as much memory as it can by clearing out what the developer considered unnecessary information.
Another point of of possible quitting is if a phone call is being received. Apps only have a split second to save what is crucial before quitting to make room for the call. Again, the developer has to decide what is truly important and what isn't to save.
The save position may not sound like a lot, but mobile devices really don't give developers a lot of room to play with and media of any type is a resource hog.
Best Answer
Your computer iTunes is waiting for the iPhone to "fix itself" and clean up from the interrupted sync.
The mobile iTunes database (and I'm sure several others) will need to be rebuilt to clean up after an interrupted sync. You can see/test this by disconnecting your iOS device in the middle of podcast or music syncing. If you then immediately start up iTunes for iOS - you will get an Updating LIbrary screen while it itdbprepserver does its work. You'll get a "verifying message" until these post-sync and interrupted-sync cleanup activities finish. Once iTunes on the desktop has "verified" the phone is ready to start another sync, it commences with the normal backup and sync activities. My timing shows the interrupted recovery to take twice the time of a normal sync completion but usually less than a minute either way.