The MGS titles are *never* huge in Japan - even with a huge install base. It should do really well in the US & Europe, but its just not the saviour people think it will be (for the PS3).
It may take some time, but I'll be surprised if it *never* makes the cross to the 360...
...
For the launch of MGS4 in Japan, here is a good starting graph. The danger is this - if retailers don't favour the PS3 at the moment, they won't order a LOT of units of MGS - it will sell out, and lose sales in the long term:
http://vgchartz.com/swlaunch.php?reg1=Japan&game1=Metal+Gear+Solid+2%3A+Sons+of+Liberty+-+PS2®2=Japan&game2=Metal+Gear+Solid+3%3A+Snake+Eater+-+PS2®3=Japan&game3=&weeks=10&weekly=1
MGS2 & MGS3 both launched at around 500k sales for week#1. Given the state of the PS3 install base, I wouldn't be surprised to see MGS4 do about 60% of these numbers - around 300k for week#1. Anything under that will be a serious disappointment.
Also notice how sales drop off 80% to week#2 - a very significant drop-off. MGS4 *may* struggle to sell 500k lifetime in Japan (a poor result for sure).