Another update. Over at the Italian blog, Archivum Liturgicum (who originally made the rumour that a decree had already been signed), they are rumouring as follows tonight, seemingly confirming the story about Vatican Radio (which I personally haven't been able to confirm in the minimal English language programming on Vatican Radio):
Vatican Radio announces the papal document on the rite of the Mass
Today Vatican Radio has announced that His Holiness Benedict PP XVI
has signed a document about the Rite of the Mass, as has been foreseen on this blog. The document will in all probability be published on the occasion Maundy Thursday.
Thanks to, Juan, for pointing this out. And again, please remember folks, this is all unofficial as of yet.