It's amazing to me that this dialogue box still exists in iTunes 10.5.2:

iTunes has found purchased items on the iPhone that are not present in your iTunes library. Do you want to transfer these items from this iPhone back to your iTunes library?

No option to see what the items are, just that things will be removed if you don't transfer them. Terrifying if you don't know what you happened to delete from your iTunes library that's still on your iOS device.