I don't recall Reeves saying this, and I believe Cage is talking out of his pompous ass, like usual. That's just me, though.
Honestly, Heavy Rain will probably be big, given Sony's support of the title, but God of War III and Killzone 2 will be much bigger. And if Gran Turismo 5 releases next year, it will crush them all.
That being said, I do believe that Heavy Rain will do much better than it would have otherwise, since the game is being published by Sony. I actually believe the game will sell better as an exclusive than it would if it were multiplat. Any third party publisher would've treated Quantic Dream like the niche developer they are in regards to budget and marketing, and this game would've sold no better than Indigo Prophecy. Sony, however, is obviously putting a lot on this title, and will probably market it very well. We've seen in the past that Sony isn't afraid to take niche concepts and try to turn them into something much bigger (the Ico titles, and LittleBigPlanet), and this is just another example of this.
Even if Heavy Rain was somehow Sony's biggest title for 2009 (which it's not, not even close), I wish Cage would be a bit more humble about it. For example, this was Media Molecule's reaction when they realized just how important their game is to Sony:







