Perhaps the biggest feature available in the iOS 4.3 update today is the Personal Hotspot -- Verizon iPhone users have had access to the feature before now, but as of iOS 4.3, AT&T users can find the hotspot feature under the Network tab, inside the General Settings app on the iPhone. Note that while Personal Hotspot makes it possible for you to share a network connection with a computer, either over Bluetooth, a USB connection, or now over Wi-Fi, there is an extra charge on AT&T, and your phone will ask you to sign up for that service if you haven't yet.
AT&T calls the service DataPro with Tethering, and it will cost you an extra $45 a month, allowing up to 4 GB of bandwidth, and extra charges beyond that for more. That's not cheap, but if you are often in places where you have to depend on your phone for an internet connection, it could be worth it.
To use the service itself, you just jump into that Settings screen, set up a password, and then connect from your computer as you would any other Wi-Fi network. The hotspot works with up to three connections via Wi-Fi at a time, so you and two friends can access the internet from anywhere, right through your iPhone.