For sure, bringing it to the 360 is a clear cost-recouping move. Remember that they announced the game for PS3 way before the current system wars shook out, and the landscape looked way different back then.
In the article KDR posted above, they said at least April 2010. I'm assuming that's for the PS3 version, which would only require straight script translation and localization (i.e. no code porting, so less programming and testing involved). Square-Enix's 2008 fiscal year ends March 31, 2009, so it looks like their plan is to release FFXIII for both PS3 and 360 in the same fiscal year (which makes sense, because there's no reason why you'd want to split the revenue from both games across two fiscal years).
I'd expect the 360 version to be released in the fall (maybe September/October). I figure it'll take a couple of months to port, and then probably six months to test. Of course, this is all just my guessing. ;-)