The enhancement was observable in Yahoo mailbox. This enhancement might be observed in other third party email providers like Gmail or Hotmail, too. However, I can not guarantee the validity of the statement as I have no valid data.
To see the enhancement, you should opt-in wall or comment notification and configure the email client software to automatically display email content as HTML instead of plain text. Naturally, your Yahoo mailbox settings should have been configured to display HTML email by default. When you open the recent wall or comment notifications from Facebook, you will notice slight difference in the visual representation of the email. The notification email now also includes a text field in which you can type your comment and automatically post back to Facebook without having to log in first. The picture below shows the newly integrated text field.