I've just created a new field with a 'yes/no' answer. Can I auto-fill all records with a specific answer?
Once you set the default value, you have to somehow trigger an update for your contact records so the default value gets populated. One way to do this might be to export a minimum of contact fields and re-import them (careful with this technique, unless you do it properly you'll create a lot of duplicate contacts in your db!).
You can set a default value in the custom field definition.