Given an array of post data to insert or update, it is easy to add a Post Format to the taxonomy array (similar to the category or post_tag data). The following assignments can be using in a conditional statement to automatically attach a Post Format to a saving post.
I’m currently using Theme Hybrid as a parent theme and have been exploring the internals with an eye to using WordPress 3.1′s new Post Formats for a lot of my content. I import my tweets from Twitter via Ozh’ Tweet Archiver; I import my bookmarks from Delicious via FeedWordPress. These imported posts were previously stored in Custom Post Types, but Post Formats seem like a better fit, especially with the hoops I jumped through to get them to appear in The Loop.
The following explores options in Theme Hybrid for using a child theme to create custom Post Formats.