Well good news from Japan, the game is really selling.
In its first 4 days, the numbers look like this (all versions of the game included)
476,334 units sold
77,208 PS3s sold
This makes it the highest selling PS3 game in Japan to date already...and I'm guessing worldwide sales of MGS4 will be in the 1.2-1.5 million range so far.
http://www.famitsu.com/game/news/1215919_1124.html