A former developer for Vampire Rain has explained that making an Xbox 360 exclusive is an easy way to fund a project for PS3.
"For developers, it's way easier to develop for the 360. Microsoft provides sufficient development funds. Support is fast and all encompassing," wrote the Japanese dev on his blog (picked up by Kotaku). Signing a one-year exclusivity deal, apparently, is all that is asked.
On the other hand, the dev continued: "Sony Computer Entertainment doesn't really provide money (maybe no money). Support is a mess and unreliable. (At the time I was involved)."
Therefore, he added, companies like Namco develop first for Xbox 360 and then port cheaply to PS3 to recoup costs. Sony won't approve a exact replica, however, so "lots" of extra content must be added.
This is often why features scrapped for the Xbox 360 launch are added to PS3 games. And, he added, why these do not appear as DLC, retroactively, on Xbox 360.