"Geoff Keighley basically confirmed this last week. According to him it is because Sony is allowing user created mods, and Microsoft thinks it could be used to undermine their Xbox Live service. He says PC and PS3 versions release at the same time, and the 360 is coming sometime later after they sort out some issues.
In fact, he was very positive on Sony’s open attitude to networked content compared to the Xbox 360 - he commented of Microsoft’s platform: “Unfortunately it’s more of a closed platform,” and even noted that Microsoft seems to be “quite negative toward user-created content” under certain circumstances Epic has encountered.
He notes that the mod community was “an essential part of [Epic’s] success”, and commented: “We would love to transfer this mod community over to the console platforms.” Of course you’ll need a PC to create levels for the upcoming UT2007 for PS3/PC, but Sweeney believes that everything will be in place so that modders can make new levels on the PC, “download them to the PlayStation 3, and distribute them online.”"
En gros, ils vont d´abord le sortir sur PS3 car ils souhaitent y intégrer un mod exclusif (le PS Network le permet) : ils sortira donc sur PS3 et PC. Il sortira plus tard sur 360 car le Xbox Live ne le permet pas. 