What Do I Bring With Me to the Store?
To get out of the store with an iPhone 3G, you'll need a credit card to pay for the phone, and your Social Security number and photo ID for the credit check that's part of the process. (The credit check is something AT&T requires prior to signing up a subscriber.) Apple's also said you'll need your current mobile phone number and password or PIN if you want to transfer that number to your new iPhone.
How Can I Tell Whether a Store Has Stock?
Apple will fire up its inventory tool -- which it used last year with the first-gen iPhone and then earlier this year after the launch of the MacBook Air -- so you can check online before you leave the house.
The availability tool will be here, and will reflect next-day's status after 9 p.m. local time for the store you're checking.
AT&T doesn't have anything similar, and in fact explicitly says that just because a store pops up on the locator doesn't mean it has iPhones in stock. "To make sure iPhone is at the store closest to you, call that store's number," AT&T said.
How Do I Transfer My Data?
iTunes is your best friend here.
Apple posted a support document last week that spells out the steps you'll take to move information from an old iPhone to the one you just bought. The short version: Sync the original iPhone to iTunes -- you'll need iTunes 7.7, the just-released version to complete the process -- then connect the iPhone 3G to the same computer and sync that from iTunes.
AT&T, meanwhile, offers tips on how to transfer data on non-iPhone phones, whether they're associated with a current AT&T account or not (download PDF) . According to that document, AT&T will even transfer contacts from a non-AT&T phone to a new iPhone at an AT&T retail store.