Just go into any Best Buy that carries the rMBP and you can preorder. I decided not to cuz I didn't think it was any different in delivery time than ordering online from the Apple Store yourself.
Anyways, just drove out 30 minutes to my local Apple store at Walden Galleria in Buffalo around 1pm and had the absolute WORST experience ever. (Someone from their store told me on Sat. that they were expecting a shipment today around 1pm.) While waiting quite awhile for a specialist, I noticed that someone had decided not to purchase their rMBP and his specialist took back the rMBP so I interrupted him and immediately and asked "May i purchase that one?" For whatever reason, the specialist hesitates, and tells me as I watched the BS unfold, "No, this one is on reserve, but I'll check if we have anymore in the back." As I expected, he goes to the back for less than 5 seconds, and then comes out saying "they're all out, and we don't know when we'll be receiving anymore." Then he does his ********* demonstration of how you can order it online. (No shi* buddy, I'm here so I don't have to wait 3-4 weeks). While he tells me this, I overhear another person purchasing his rMBP and his specialist tells him, "you're very lucky, we just got 2 in this morning and you got the last one." Clearly, I saw the two boxes of the rMBP, the one from the person who decided not to purchase it, and the other "lucky guy." I don't understand why they wouldn't let me purchase the one that the first person refused to buy cuz clearly there's no "reservations" on these new rMBPs.