I don't agree with CrisKre's numbers, but I do think this will be an interesting case study.
I think it's fair to say that MGS4 seems to be getting a bigger advertising push than MGS3 did (likely by a substantial margin), and there's no denying that it's gone from one of a host of major Playstation exclusives to being fairly isolated: there hasn't been a game of this size for months now, and probably won't be again for months on end.
Therefore, the game has free reign of the PS3 for a few months here, has a bundle working for it, has more advertising, and so forth. Absolutely everything is setting it up to sell better than before, except for the platform it's on. Which is why it will be interesting to me: if the game does sell substantially worse, there will be absolutely no argument that the game wasn't hindered by the platform of choice. On the other hand, if the game does manage to equal the sales of MGS2/3, that would indicate that a PS3 game can live up to its previous iterations, albeit only if absolutely everything aligns perfectly and Sony helps you market it.