Today the Apple site was updated with a nice "Buy iPhone 3g" button suggesting that you can now buy the iPhone online. Don't get excited you can't actually order a new iPhone from Apple via the internet. What you can do is pre-qualify and choose out options like your AT&T rate plan so once you get to the Apple store you only have to pick up your iPhone.
While this may save a few minutes for some I don't really see the point. Something like this would be extremely useful for a new product launch day where lines are extremely long and iTunes activation crashes -- of course then Apple wouldn't have all the hype and fanboyism outside their stores and the excitement of the the launch wouldn't be nearly as high.
If you find it easier the option to get some of the paperwork done online before you pick up your new iPhone is there for your convenience. Enjoy.