Pope Benedict's letter to bishops accompanying the Motu Proprio
The Motu Proprio itself (in Latin)
As of now, the MP has been released in Latin only. Perhaps after the sometimes huge discrepancies in the translation of the recent Apostolic Exhortation Sacramentum Caritatis there's a whole new level of proofreading going on. So be patient!
In the meantime, Fr. John Zuhlsdorf has an excellent explanation of "what it all means" on his blog, What Does The Prayer Really Say.