NullRiver's NetShare tethering application for iPhone worked like a charm with my MacBook Air after lumbering around like a big dufus with the set up. The browsing speeds were not as cumbersome as I expected using 3G which was a sweet surprise. For $9.99 this iPhone application that allows users to access the internet from their laptop computers through wireless tethering wherever they get service on their cellphone carrier’s data network, is well worth the price and could come in handy when traveling. I suggest keeping your iPhone plugged in so it won't lose it's charge so quickly. NetShare is an approved Apple app not requiring a jailbroken iPhone.
Engadget posted a very simple walk through video that helped me get connected in a snap after my own bout of expletives. Adding the matching Subnet Mask on my iPhone turned out to be the missing link. A necessary tidbit that the NetShare folks left out of their instructions. Safari is the only browser that works with the NetShare app so don't let that one trip you up if you're a Firefox freak. Now for the bad news...
AT&T lists under its iPhone Prohibited and Permissible Uses that tethering connections cannot be used. This may explain why the NetShare application was mysteriously abducted from the App store shortly after several websites announced it's arrival. Why it suddenly reappeared is also an unexplained X-Files style mystery only Apple knows the real answer to. I spoke to an AT&T customer care person who immediately confirmed the "no tethering" policy after keeping me on hold briefly, also verifying they do not currently offer any specific tethering usage plans.
"Furthermore, plans (unless specifically designated for tethering usage) cannot be used for any applications that tether the device (through use of, including without limitation, connection kits, other phone/PDA-to computer accessories, Bluetooth® or any other wireless technology) to Personal Computers (including without limitation, laptops), or other equipment for any purpose. Accordingly, AT&T reserves the right to (i) deny, disconnect, modify and/or terminate Service, without notice, to anyone it believes is using the Service in any manner prohibited." as posted on the AT&T website. [link]
NullRiver may have launched the start of an epic battle for fanatical iPhone users unwilling to settle for lame tethering limits while paying a premium for unlimited data. Cult of iPhone faithful should proceed with the knowledge of clear risks to your data plan and AT&T service. Now you know.
AppleInsider: 10 Step NetShare Set-Up with Screen Shots
UPDATE: NetShare has vanished again from the iTunes App Store into some alien spacecraft for a proper probing. It's 1:32 AM on August 2nd