Is it possable that Software Update or some other download is how they will confirm you have Leopard to qualify for the $29 upgrade?
Maybe even just Download SL the installer realizes you have Leopard & then they send you the disk for later use? Or even a "Pick up at the Apple Store" option?
I don't think they're going to push SL over a software update. It would be over a few gigs and totally take down Apple's servers easily.