I could have sworn I saw this for sale (not the MGS2 & 3 collection) a week or so ago which seems to have been according to plan http://www.xbox360achievements.org/news/news-12688-This-Week-on-Xbox-Live-[XBLA-and-DotW].html

But now there is a listing of the game on the Games Marketplace but it is not for sale. What gives?

Not sure why this happened at all but just wanted to link you to a NeoGAF thread that is on the same issue. Probably will see response there before here I'd guess.

http://www.neogaf.com/forum/showthread.php?t=489675

Was thinking about buying this too so hopefully it's temporary.

EDIT: From that NeoGAF thread, it seems there is some issue where the game just fails to start at all and stays at a black screen:

http://www.neogaf.com/forum/showpost.php?p=41697381&postcount=8

i actually had guys on my friends list playing it. Bought the HD collection, haven't tried peacewalker yet.

peacewalker was pulled do to the fact that Microsoft didn' got a fataly flawed version of the game that is missing some parts to it, got LOTS of complaints about it, etc.etc. and sont worry if you already bought it you will be able to download it at no extra charge...since hey its the same exact game after all.

That's too bad, because Peace Walker is a fantastic game, really fun with a bunch of cool systems and extra stuff to do, even if the sort of boss fights against tanks and helicopters suck pretty bad.