I'm coming over from Verizon, but I've done this before. Last time the 3g came out, I switched from Verizon, but I went and waited in line at the At&t store. All I did was buy the phone, come home and set it up through iTunes. I was a breeze.
I cannot remember all the details, but you basically port your number over. I believe if you pre-order, it would be done online when you pre-order, or maybe when you actually get the phone and do it online? I'm sorry I'm not much help. I do know that you don't even have to call Verizon to tell them because once your number gets ported over, it closes your Verizon account ( unless you have more than one line on Verizon ).