Wow, lots of Killzone 2 flak here so I'll guess I'll add my 2 cents.
One of the main things in selling a product is to offer convenience to the customer. In fact, one of console gaming's selling points is convenience of plug-and-play compared to PC gaming. You want to make it as easy as possible to make your product known and experienced by as many people as possible, right? Since game demos are the norm in this generation, offering such exclusivity is really counter-productive if the purpose is promotion. Any "deal" with Gamestop was, in my opinion, a bad decision on Sony's part because it only alienates subsets of users who don't pre-order or go to Gamestop. This type of physical alienation isn't a good marketing decision in any circumstance.
Joining the exclusivity club can be good way to promote a game, but you have to make it convenient. Download this wallpaper, watch this video, or play the demo to join the Killzone hype club is a better way to get people to join your group, not go into Gamestop, lay down 5 dollars, go home, enter code, download demo. It's too many steps. Gamers are lazy, parents are lazy, kids are lazy.
And besides, If I'm pre-ordering a game, I want something other than a

ing demo. I'm going to be playing the damn game in 2 weeks, what the hell do I need a demo for if I'm already buying it? Give me something substantial that I can hang onto or put on my shelf, like a zelda compilation. Something with meaning is a much better driver of pre-orders than something that becomes worthless when the game comes out.