I'm using Gmail for Contacts, Notes, Calendar, and Mail sync.
Since I only have one account, most of these apps don't have the top level selection page which let's you choose which account to use. They always use my Gmail account, which is exactly what I want.
The one exception is Contacts, which allows me to tap "Groups", and then has an option "All on My iPhone". I want to disable this feature, so that I don't accidentally add contacts that aren't synced with Gmail. The Notes and Calendar apps do this perfectly... I can only add notes and appointments to my Gmail account, not to the phone itself.
I'm using Gmail's Exchange sync, by the way.