Everyone assumes MGS is gonna sell huge, yet "Snake Eater" had a lot of trouble at first, and it still fell below what either MGS or MGS2 sold.
But then again, this is a whole new graphics engine; so a little more excitement from the masses is likely.
But what masses?
There are far fewer owners of PS3s than there were PS2s, and most probably won't get one just for MGS4 (not in this f'ed up economy). The target audience is much smaller this time.
At the same time there isn't much else to be this excited for on the PS3, so maybe they will gain new players from that competition lull.
But... You take a look at the sales environment of past MGS releases and they were all released while their respective systems were experiencing healthy growth and stability. PS3 has none of that. MGS4 is charging out on a whole different battle field, and it definitely has a battle ahead.
I also think people are losing interest in the series as it becomes increasingly experimental in its gameplay... and... well... weird. Despite it's huge sales from the past, MGS still remains a very off-beat franchise; probably because it has one of the strangest lead producers in all of the industry.
Kojima-san may be a genius, but that comes with a very different mind.