There has been much disappointment with AT&T for not enabling tethering the iPhone to your laptop for Internet connections.
As a prior Windows CE "Smart Phone" user I have found this particularly frustrating as there were no restrictions on tethering the Windows CE phones. It was a lifesaver for us when traveling and staying at campgrounds that did not have a Wireless network to use.
The release of the new 3.0 iPhone OS, includes tethering BUT it is not functional in the U.S. as the Configuration File installed on your phone by default, does not enable tethering.
With the release of v3.1, and later, Apple has disabled the ability to Tether. It will appear that you still have it after updating your phone OS, but the first time you try to use it, it will fail and the "Tether" option will disappear from the Network configuration window! If you need to Tether, do NOT update the OS.
I have always been reluctant to Jailbreak my phone, although that approach has provided tethering means for some time. With the release of OS 3.0, however, a number of people have published non-jailbreak methods of enabling tethering. Most require you to download a Config file, or modify your existing default file using iTunes.
All of these methods seemed to me to be fraught with unacceptable risk, and all I examined seemed to have blog posts here and there indicating that their implementation caused Voice Mail or other iPhone features to break!
Recently, however, I came across an approach that seemed more benign that any other I found. I tried it, and to my pleasant surprise it was very simple to do and produced perfect results that enabled tethering and did not break anything else that I could discover. I have tethered twice now for brief periods, with higher than expected speeds and no problems.
That said; NOTE that I have only used tethering in brief situations where it was the only means to access the Internet with my laptop. I have NOT used it as a replacement for a cellular modem. I suspect that AT&T is worried about that kind of abuse and thus has delayed enabling tethering until they can determine an associated rate plan and beef up their network capacity.
Accordingly I urge anyone trying to tether to NOT ABUSE the privilege at this time. After you enable tethering, if you review your next wireless phone bill, under the Data Plan section, you will see "phone" data sessions listed, as well as "WAP.CINGULAR" entries. These WAP entries reflect your tethering use. If you use it sparingly, you may probably be OK, but if you abuse tethering, it may well be flagged by AT&T's computers. Thus use tethering at your own risk as there is the potential for AT&T to hit you with unexpected charges.
Browse to the above noted link with your iPhone and then click anywhere in the AT&T entry (circled in red).
That's all you have to do. The link installs a new Mobile Configuration file on your phone.
You can now go to Settings>General>Network and you should see that "Internet Tethering" now appears as an Option. If it doesn't, then do a Reset on your phone (hold down the power button until the "slide to turn off" button appears). After it has powered down, then turn the phone back on (hold down the power button until the "Apple" appears).
Once tethering is ON, you can connect your iPhone to your laptop via USB or Bluetooth. Go to your laptop's Network Settings window and select the phone as your network connection method and you should be good to go.
When you are done, you need to turn tethering off on the iPhone before you can run applications again.
Note: Make sure your phone's data plan supports the data volumes you will use, otherwise you might get a very expensive and unpleasant surprise when your next phone bill arrives (could be in the thousands of dollars. . . no kidding!).
Also note that the above referenced link was, and remains, totally unknown to me as to credibility, integrity, etc. Use at your own risk. All I can say is it seemed to work fine for me.
The following has been added to the original post . . .
To remove the Tethering capability, go to Settings>General and scroll all the way down to the bottom of the screen.
There you will see a line that says Profile and it should contain "US AT&T" in it on the right side.
Tap on the Profile entry. It will open up a window with a description of the Profile and a Remove button (image to left).
Tap on the Remove button and when it finishes you should be back to your original (factory) default profile. To be sure, I would also suggest doing a Restart (on/off sequence) on your iPhone too.