This version is only compatible with XenForo 1.2.
This version still includes the Preview button added to the thread view, but it now also adds the Preview button to the conversation view.
In addition to this, it's really straight forward for add-on developers to add a Preview button to their own add-ons.
All they need to do is call the quick_reply template with a $previewurl parameter and add a button with a PreviewButton class.