I'm quite sure that unless PS3 installed base becomes >> XB360, Halo-like FPS' will still sell better on the latter, but the sales boost given by PS3 port should be more than enough worth the effort anyway, this gen good games ported with care sell well. And I could be proven wrong, during the last months God of War and MW2 helped triggering a lot of love for PS3 amongst American hardcores.
Let's just hope Activision isn't even vaguely tempted to treat Bungie as it's doing with IW, though...