It doesn't seem like fallback to a default language has been implemented the way i18n tables work now.
Currently, once you set a locale you will not get back any content if there's not a row in the database with that locale set for the query you're executing.
What happens to users who view a blog (for example) in a locale where only half the posts are translated? Not showing the other posts at all hardly seems appropriate. :)
This is critical functionality for i18n tables, and I think a solution in Symfony is much better than each user rolling his or her own.
More information here:
http://www.symfony-project.com/forum/index.php?t=rview&goto=21705