In a transcript of the IRC chat posted today, Rein told BeyondUnreal that synchronizing the PS3 build and PC builds of the game to allow cross-platform play would hamper the company's ability to respond quickly to issues that might crop up on the PC side (read: cheats and exploits.) His statement:
"We looked at how this would impact our ability to respond quickly to things happening at internet speed on the PC side of the equation and realized that would not be in the best interests of our very loyal PC user base because we would constantly be holding on to updates to wait until they passed cert on the console platform,"Rein did not rule out future cross-platform UT3 compatibility, after the game stabilizes a bit and gets some early updates.
[Via IGN]