Because it goes without saying that this will be hot. It doesn't bode well for a manufacture/retailer to say "Your iPhone is guaranteed all day.*"
* BUT THE LINES ARE GONNA BE R-I-D-I-C-U-L-O-U-S if you aren't in line by the time you got this e-mail YOU AINT GETTIN' 1 HAHAHAHA! LOL TOOO BAD
But maybe i'm wrong.
Also, of course the pick up line is going to be longer than the walk up line, but it doesn't matter because the walk up line is going to be moved to the BACK of the pickup line. So there are alot of different metrics to consider. As a note i'll mention that I actually preoreded two iPhones when I only needed one and they called me up today to ask if I needed two. I told them no and they said they will return the other one to stock, (so someone out there will score a 16 gig black thanks to me). So take the fact that they have sufficient pre order stock, plus an undisclosed quantity for walk ins who knows. I do know that If they run out of walk in quantities and customers are there with cash in hand they will start picking from the reserve pile. Can't turn customers away when you have stock 'set aside' but unpaid for.