An Email Insider article mentions that hotmail should be releasing pretty soon a new inbox placement metrics system based on the user engagement.
It's true this story has been around for almost a year, but knowing what Gmail just did, rolling out their new (Beta) priority inbox, it makes sens that hotmail wouldn't want to be left behind and that it should probably speed up the switch to a more user oriented inbox placement.
On a more general point of view, I reckon this user engagement measurement system will probably in the end be added to the spam filtering systems on a global scale.
After all it's probably the most sensitive and logical way to filter spam.
