Follow this logic if you can. Killzone 2 can be preordered. Anyone who bought the game, bought as in already committed their money to it, will be able to test the game via the demo starting February 2nd. The rest of us have to wait. What? Yeah. Here is what Sony says:

Offering a playable demo to motivate preorders speaks to our confidence in the appeal of a game such as Killzone 2. We take this approach when we are convinced that the experience will cement a consumer’s interest in purchasing. Retailers will then merchandise those demos within their stores and online to maximize their visibility. Through this process, consumers are often times able to reserve their copy of the final game, which provides an incentive to purchase and helps seal the deal. We also offer demos on the PSN, which we have done in the past with great success, and will continue to make that available. Killzone 2, for example, will have a downloadable demo launch in North America the week that the game is launching, for consumers who prefer that option.

In summation, anyone who already bought the game can try it out, but whether they love it or hate it, they still own it. For those who might want to buy it, but would like to try a demo are shit outta luck until the day before release. This makes less sense than the Chewbacca defense.