Metal Gear Solid: Peace Walker hasn't been on Games on Demand for long, but it's recently disappeared from the Xbox LIVE channel without much explanation.
Microsoft has issued statements to both their US and Japanese support sites. Neither one actually explains what the issue is, however, or why the game was removed. On the English forums, an admin responded to the community complaints, "We are looking into this presently. I don't have any information at this time. Hang in there!"
If you take a look at the Japanese forums, Microsoft's statement says more or less the same thing, just in a different tone. The translation goes something like: "We are sorry for the inconvenience to everyone" and "the resumption of delivery is being addressed now, please wait for a while."
So if you're still interesting in grabbing Peace Walker of Xbox LIVE, just hang in there.