http://www.square-enix.com/jp/company/j/news/2007/download/release_070125.pdf
Some next-gen Square RPG goodness possibly going multiplatform? That's the only good reason I can think of for choosing UE3 over their internal engines. UE3 appears to work equally well on all platforms (PS3, 360 and PC.)
This press release just came out today, so an English source may take some time.
Basically it says that "We [Square-Enix] have acquired the Unreal Engine 3 middleware from Epic Games to help in the development process of video games"
They go on to explain who Epic is and what the Unreal Engine 3 is and how it is usually used in the United States. They say that they will be using the latest version of Unreal Engine 3 which is known for its excellent balance of quality and development efficiency.
Some next-gen Square RPG goodness possibly going multiplatform? That's the only good reason I can think of for choosing UE3 over their internal engines. UE3 appears to work equally well on all platforms (PS3, 360 and PC.)