Jobs said "ships by the end of this month", not "available at the end of this month", meaning that they will have to be "available" before they can "ship", therefore they should be up for 'pre-order' before the end of the month.
It's not technically a pre-order, per se, but since it'd be an order for a product that hasn't shipped, that's what I'll call it.
I guess what I'm trying to say, is they don't have to announce an order date. One day, the store will simply go down, and come back up with the white iPhone available to order, with "Ships: By July 31st" next to the picture.
Of course, with limited supply I think the date will slip to "Three Weeks" or perhaps even longer within a couple hours.