The official announcement won't be until the end of the week, but this is from a reliable source.
Following Symphogear's musical inspirations the producers have searched for new ways to market the show and so it has been decided that Hatsune Miku will be appearing as a character in Symphogear G! She will have her own songs and designs made exclusively for the show.
The key visual on the website has been updated:
Nah, I'm just kidding with you. Happy April Fools'! (≧∇≦)/