Petroglyph could be developing a MMO title according to a jobs listing which has appeared on the American site Gamasutra. The position is for a MMO Game Designer who is responsible for understanding, refining, maintaining, and disseminating the game design vision of a Massive Multiplayer Online Game.
The game is likely to be in the very early stages of development so we may not hear anything more about it for awhile.
Petroglyph is currently working on Universe at War: Earth Assault which is almost finished. We've been given a chance to try out the beta version of the game so expect a preview to go live on Strategy Informer within the next few days.