Disagea has always been Playstation.
However, the company that made it, Nippon Ichi, has entered into a contract of sorts with Microsoft and the X360 to make games for the X360. So I have a feeling that Disagea will probably be PS3/X360.
FYI, Disagea does best in the US and PAL territories, not Japan...Which leads me to believe it might go X360, despite no confirmation.
Back from the dead, I'm afraid.