I believe you need to buy through a 360 or something. Only for BC games.
Minor word of warning - you can't use Microsoft/Xbox credit to buy these BC games. I bought £10 and then couldn't use it, d'oh :p
I was going to post this myself - they never seem to show this on the list of Sale games when i'm browsing on my Xbox, for some reason. Heeeeaaaaaaat
Always regretted never platying MGS 3 as it's such a classic. Thanks OP, ordered! :D