The rumor is plausible, but only because what it states is logical. That being said it could just be good intuition, and in no way might be legitimate. In other words the source could be bogus, but the outcome could be the same.
Neither Nintendo or Microsoft have been very open with their high end exclusives for next year. Only a supreme idiot would suppose that they do not have a deck stacked with surprises at this point. Even Sony dropped bombs with what at the time amounted to six month notices before the delays crushed their holiday lineup.
Microsoft has had ample times to seed projects designed specifically to counter the PS3 lineup. This assumption that they will allow themselves to effectively rope a dope with Sony this year is pretty naive. Honestly its wishful thinking when someone does not acknowledge that Nintendo and Microsoft are not hiding killer games to be whipped out on a moments notice.
What exactly isn't plausible about this rumor I wonder. Microsoft certainly has a strong interest in undercutting Sony exclusive titles. They would probably front many million dollars to see it happen. Graphically none of the consoles has truly been tapped out yet. Most consoles do not display their true power until later in their lives not in the second year on the market. Developers when it comes to the 360 have a year long edge to make better use of the hardware.
I would say this if the rumor is true we should know within the next few weeks. Metal Gear is looking towards a summer release, and release lists need to be finalized. Magazines need a four month window, and more importantly you do need to give the hype machine time to warm up. So any later then late January or early February, and we will know whether their is a stealth bomber aimed at Metal Gear. Would make for an interesting year.