The BIG THREE have information on the release of the Motu proprio. According to an Italian news report cited by Rorate Caeli, Father Z and NLM, the Motu proprio and its accompanying letter is slated to be released before Benedict XVI goes away for the summer. Rorate Caeli states that the Pope's vacation starts on 9 July.
The Motu proprio is for the freeing up of the Mass of St. Pius V as edited and set out in the 1962 Missal. It is speculated that it will allow priests to say Mass according to the previous form without needing authorization from their ordinaries who might otherwise object.
Keep reading the three linked-to blogs for more info and interesting comments.