Rumours of MGS4 being timed exclusive for the PS3 have been running around, which, given previous trends, isn't out of the question.
Kojima cirtainly isn't a sony fanboy, as other itterations of the series for other consoles have proven, and it was only bad sales of MGS2 for the xbox (due to bad sales of the console itself) that kept MGS3 off the machine.
However, MGS has always sold better in the US than in Japan, and since the 360 has more market dominance, logic would dictate that sony would have to sink a lot of cash into keeping MGS4 exclusive.
It's impossible to tell if there's any weight to the rumours that it's already being developed for the 360, there's nothing official but people have been quoting 'inside sources'.
Basically, if MGS4 is the only thing that would make the difference between a PS3 and a 360 then you might want to wait for news of the exclusive status of the game before you sink your cash.