you should call at&t...thats what I did...they will check to see if the number is able to be ported over. Since it is verizon shouldn't be an issue. If I were you, I would wait a week and a half since paying $36 to port the number after it has been 30 days with your new account is a waste of money. When you get the phone, and I assume a new #, you will need to ask for the telephone number to call to get the number ported. It isn't the same as regular customer service at At&t and the at&t customer service rep didn't transfer me.
The transfering # dept was very specific with the fact that you have up untill 30 days to port the number free of charge. After that it is a $36 fee.
I was an idiot and had about 45 days left and I am not going to port my number. I had been sitting on $500 in apple store gift cards going back to last Christmas. I was going to be on the road pretty much every weekend for the next couple of months, and I wanted the internet access and ease of use that the iphone is giving me. It would have been nice to port the # over in store or at least keep my verizon cell phone number, but IMO the pro's of giving up the number outweighed the cons.