My bet is that Microsoft has a timed exclusivity agreement in place. The terms of such an agreement would prevent any mention of other versions until a minimum amount of time has passed. So if there's a PS3 version in the works, it'll be at least two more months before you'll hear anything about it.
As for whether a PS3 version actually is in development, that depends on whether Square-Enix thinks it will be profitable. Star Ocean didn't have the serious technical issues of The Last Remnant, which would be even harder to fix for the PS3 version than for the 360 version. Star Ocean has also sold better than The Last Remnant, and has a fair-sized Japanese fanbase. For those reasons my bet is that a PS3 version is in development, and you'll hear about it in two to five months.
