Nice, the first WP 8.1 phones are available, like the nokia lumia 630. Since all WP 8.0 phones can be upgraded to WP 8.1 the „real“ update (not the developer preview version) should be available for everybody soon. 🙂
My app Week View already support windows phone 8.1 and the WP 8.1 version is available as a free update for all users of the WP 8.0 or WP 7.x version.
Next thing to do is porting LockScreen calendar to WP 8.1 too and add some new features to Week View 🙂