I'm sure many readers may think this is a damage control article, but rest assured, it's not. When Microsoft secured a multiplatform release for Final Fantasy XIII on the Xbox 360, it definitely damaged Sony (and Square Enix to a certain extent). However, in hindsight, it may not be a bad thing for Sony that Final Fantasy XIII went multiplatform. Let me explain why.
1. At the moment Final Fantasy XIII is secured as exclusive to the PlayStation 3 in Japan, although it has been rumored that Microsoft may announce a Japanese release of FFXIII on Xbox 360 at E3. If this happens though, it will not be day and date; there will be a severe delay. Previous Final Fantasy games have consistently shifted approximately 2 million units in the first 5 days, each time with an install base of about 4 million. Unusually, Japan is the region where Final Fantasy performs the best, so having exclusive rights to a Final Fantasy game there means that it will definitely push console sales up. Also, bear in mind that by the time FFXIII is released, there will almost definitely have been a PS3 price cut, so expect good sales.
2. When the game releases in Europe and North America, it is said to be releasing day and date on both the PS3 and 360 (according to Microsoft, not Square Enix). This means that most likely the Xbox 360 version of the game will outsell the PS3 version in North America, but probably not by much. Multi-console owners will choose the PS3 version of the game, as it will be on a single Blu-Ray disc rather than (most likely) 4 DVD discs.
3. If people buy and like the 360 version of the game, they will realise that the whole FFXIII universe (Fabula Nova Crystallis) can only be played on Sony formats; Final Fantasy Versus XIII exclusively on the PS3, and Final Fantasy Agito XIII exclusively on PSP. This should lead to an increase in sales for Sony formats, as well as increasing the number of Final Fantasy fans.
4. Sony owns an 8.26% (approximately) stake in Square Enix. This means that 8.26% of all profit, from each copy sold, whether on PS3 or 360, goes into Sony's pocket. So people purchasing the Xbox 360 version of the game, however hardened fanboys they are, will be supporting their greatest enemy in the console war. (Source)
5. PlayStation 3 owners will be getting the better version of the game. Square Enix have said many times, that once the PS3 version of FFXIII is complete, it will then be ported to the 360 whilst the PS3 version of the game is localized. Take into account that FFXIII is being optimized for PS3 first, and you realise that the Xbox 360 version will look (at least) slightly worse (PS3 to 360 ports tend to look only slightly worse compared to 360 to PS3 ports). (Discussed here)
http://www.ps3center.net//news/2768/ffxiii-going-multiplatform-how-does-it-really-affect-sony/