I recently published a new book via Kindle Direct Publishing and I don't think I had a choice as far as the format goes. That is, I don't remember being given the option to publish as mobi7 only, KF8 only, or mobi7 and KF8 combined. I think Amazon automatically publishes new entries as mobi7/KF8. Books published prior to KF8 that have not been updated since KF8 was released should still be mobi7, or so I assume.