I could be wrong, but I believe that Metal Gear Solid is a series which has seen a large portion of its sales come from the "High Profile Game Effect" ... Essentially, this can be explained as "Whenever you have a popular system, high profile exclusive games released for that platform see a boost in sales because people who aren't (normally) interested in the game buy it because it is high profile on a system they own"
A good example of this effect would be Mario Party 8 which has outsold all previous Mario Party Games even though it is one of the worst Mario Party games, and it could be argued that its success is due to the Wii's popularity more than anything else.
If I'm correct, Metal Gear Solid 4 will only provide a moderate boost for the PS3 which is short lived because a lot of people who previously bought Metal Gear Solid games will not be willing to buy a system to play the game.
