Recently, Apple has started offering would-be iPhone 3G buyers an opportunity to save time in the store and purchase an iPhone 3G online for in-store pickup (and AT&T activation).
You first tell them whether this is a new single- or dual-iPhone purchase for either a new account or an existing AT&T account. You then either enter new billing information or provide your current AT&T account data and select your rate plan and store location. When you arrive at the store, your old number will be automatically switched (if you migrated it to AT&T) or your new number will be activated and you should be in-and-out in the blink of an eye.
When I was switching my plan from my old iPhone to new 3G this weekend at my local AT&T store, availability was almost zero on the popular 8GB model due to the power adapter recall (no replacement stock yet), so this may be a great option to ensure that what you want will be in stock.
Have an iPhone 3G Apple Retail or AT&T store story to share (good, bad or ugly)? Drop a note in the comments!