What's next is a rather interesting study in group hysteria, as every meter of travel that their iPhone moves becomes reason to post a new thread. Once tracking numbers go out, there is this rather interesting tracking of which UPS cargo flight their phone is on, comments about 'hey, plane buddy, our phones are in the same cargo hold right now!' and so on.
But in the end, what's next is this: if your confirmation says it'll be delivered on the 19th, you'll receive your phone, via delivery, at some point on the 19th. You'll then enjoy your phone. This will be true whether your phone ships tomorrow or on Tuesday. If it's scheduled for the 19th, it's almost certain you'll get your phone on the 19th. Some phones ship out earlier because there's only so many phones you can fit on each plane.