Although there are millions of user that are on OSX Leopard, how many do you think actually are checking and watching for this update. We are the select great population that are monitoring this (Mac Rumors Members and such). Not all the mom and pop users that own a Mac. I know the initial server load would be high, but that is only because all of us would be hitting it. Others would trickle in as their update software does random checks for updates when they have their Mac's turned on.
I just don't see the staggered push happening, but who knows it's up to Apple.