Sega's Japanese site for Sonic Unleashed--known as Sonic World Adventure on the other side of the ocean--has the news of a delay from the game's intended December 18 Japanese release all the way to a nebulous "spring 2009."
Citing quality concerns, the Xbox 360 and PlayStation 3 editions were held back, while the Wii version is unaffected by the delay. No mention is made of the planned PlayStation 2 version.
Shacknews has contacted Sega of America to find out about changes to the North American release.
Many thanks to Siliconera for pointing the way.