It wouldn't matter much.
I mean, 360 owners were all trying to doubt the importance of this game.
And the game has essentially done all it's needed to do for the PS3. It pushed major consoles, pwned 360 in June NPD by 2:1, and made people buy a PS3 for this game.
It could come out for the 360, but it would do nothing, and set no precedent for the future for the game being multi-plat, as the game wouldn't sell that well.
But leaving the hypothetical world, and entering the real world, it could be very hard to virtually impossible to make this happen. References to blu-ray and Sony would need to be cut, and apparently a lot of game content/cutscenes to make the change from 50GB to 9GB.
I know some would argue that the game was copied multiple times because of the read speed of Blu-Ray or some nonsense like that, but no one knows if that was/is true or not. And even if it's not true, who is to say how many times the game was copied? If it was once, it would still be 25 GB. If it were twice it would be 16.67 GB. If it were three times it would be 12.5 GB. If it were four it would be 10 GB. And if it were copied five times, it would finally be 8.2 GB. I'm no expert, but I don't think that the same data is on the disc six times.
So will it happen? Probably not. If it does, what will it do?