It's not Sony that sets up the preorders but the retailers themselves. Most are holding off on taking any preorders for the PS3 until they know for sure how many units they will have. No one wants to see another fiasco like the XBox 360 launch with people waiting months for a preordered, fully paid unit.
We should be seeing preorder plans starting up within the next few weeks, maybe mid-October or so, at a guess. All I can suggest is keep your eyes open and jump on it quick when you find someplace taking them. With only 400,000 units for all of North America on day 1 they will be spread very thin.