Metal Gear Solid: Peace Walker has vanished from Xbox Live's Games on Demand service.
According to That VideoGame Blog, Peace Walker arrived on Games on Demand some time ago and was listed at $20.
Reports suggest the downloadable version of Peace Walker suffered some fairly substantial issues and may have been pulled to correct them, but we haven't heard anything first hand. We'll ping Konami for a heads up. Here's hoping we're not about to have another Silent Hill HD collection scenario; the Xbox 360 version turned out to be irreparably buggy, but Konami offered free games as some consolation.
Peace Walker originally released on PSP and was given the HD treatment for PlayStation 3 and Xbox 360, but was not included in the Vita version of the collection.