Downloadable prologue game Dead Rising 2: CASE ZERO will be exclusive to Xbox 360 ahead of the main title's release this summer.

Set in the period between the two Dead Rising instalments, CASE ZERO introduces new character Chuck Greene as he battles through zombies in the desert locale of Still Creek. The download is meant to bridge the story of the two games.

No release for PS3 or PC is planned, though both platforms are getting versions of Dead Rising 2 proper. A price for the Xbox 360 download has yet to be set.
Dead Rising 2 will be available for Xbox 360, PS3, and PC on August 31.

Source: Dead Rising 2 prologue Xbox-only - News - play.tm