Well, its very much confirmed that the Astra Militarum is the new Imperial Guard codex, leaving Militarum Tempestus as a codex supplement. This is exciting, as the leaks apparently come from the upcoming White Dwarf, and we are looking at a very cool and fast set of release for the Imperial Guard and their allies.
Please take these as rumors, as we have not had an official announcement yet.
here is a link to some of the leaks running around right now as the flood gates have been opened.
http://millests.blogspot.com/