According to a Japanese developer blog the game Valkyria Chronicles was originally planned as an Xbox 360 exclusive game. When the game was announced however, development had already switched to the Playstation 3 platform. According to publisher SEGA the game has been in development for over three years, and had switched platform early on in development. The exact reason why the developer switched to Playstation 3 was not mentioned.
Valkyria Chronicles is currently slated to hit store shelves this Fall exclusively on Playstation 3.