[quote name='mykevermin']MGS2 sold around 7 million copies, which would be 5.8% adoption rate if we use the current install base of PS2 consoles, 120 million (which may be a touch high, even).
The % becomes much higher, of course, if you look at the # of consoles sold by the time MGS2 came out. According to VGChartz data (I'm sorry, by the way), they sold around 62 million by the end of 2003. Since MGS2 came out in 2001, it is a safe assumption that they sold the bulk of those 7 million by that point. Which would make an 11.3% adoption rate.
That said, MGS3 didn't fare as well as MGS2 - additionally, there were more consoles in the world at that point. As a result, it would be unfair to take MGS2's adoption rate and treat it as a constant % we should reasonably expect MGS4 to sell at. I don't think we have a

in' clue how well or poorly it will sell.[/QUOTE]
I like your train of thought here. I think there were lots of factors built into MGS3 selling less. Not only the strong number of games coming out, but the game wasn't nearly as hyped. Remember MGS2 was E3 game of the year, and was pretty much the anticipated game of the year (as big as Halo 3's hype in the video game world). Not only that, but you had the Raiden fiasco along with substince, and a lot of people felt burned (I know MGS2 was the first game I paid full price for after moving out of my parents house and having my video game budget cut way back, I remember swearing I'd never buy an MGS on launch anymore if I would end up with the inferior version. (on a side note, I actually think MGS has driven me to avoid buying games at full price for the most part).
I think MGS4 is a bit different though. First off, it has quite a bit of hype behind it. Secondly, you just don't have as many options on the PS3 right now. When it launches, it will dominate discussion board conversations for at least 2 weeks. But, on the flip side, I know that many people buy the PS3 as a Blue Ray player only, so how many PS3 owners actually buy more than 1 game right now? It wouldn't surprise me if 5-10% of PS3 owners barely buy any games, and that will definitely effect sales here.