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.
Best Answer
Unless you manually go to that "All on My iPhone" section and add a contact, contacts would be created by default on the account you're viewing, i.e. Gmail. Just keep the Contacts app viewing only the Gmail contact list and you should be fine.iOS5 now has a Default Account option under Settings > Mail, Contacts, Calendars > Contacts (section) (Credit to Peter for the update)