Atlus fans already know that almost all of the publisher's games have been released as of now. So it's not unexpected that the publisher would send another tease to their faithful, which is just what has happened tonight.
The teaser image has a large "G" on the front of it and states that the game's name starts with that letter. While Atlus is releasing Persona 4: The Golden in the second half of 2012 in Japan and it does contain the letter it wouldn't fit under this clue.
One recent Atlus-published game in Japan does ring a bell, which is the Sting-developed game Gungnir. The title is part of the developer's Dept. Heaven series of titles which up until now have all had their main titles localized by Atlus's North American branch (excusing the Yggdra Union spin-offs).
The publisher noted in the email that they would give out another hint on Facebook but didn't state exactly when this would occur.