Well, GameSpot's article from yesterday turned out to be just a blasphemous rumor. Here's the poop straight from Matt Soell of Bungie:

Well, actually....that whole confirmation thing on Gamespot isn't true, in the strictest sense of the word. Sony hasn't approved Halo for PS2 yet, and while obviously we and Take Two would both like it to happen, it has not happened yet. Rumors are funny things. :-)Obviously we think Halo would rock on PS2 but apart from signing up as a developer, we haven't done anything towards getting the game on that platform - the first step of which is going to Sony and saying Here's what the game will be like, are you cool with this? Take Two, for that matter, also has done no work in this regard.

There you have it. As Ferr says, believe nothing that you hear, and only half of what you see.