OK, so we know it is a big no-no to tether another device (say a netbook or notebook) to the iPhone's 3G wireless data service without an authorized service plan from O2 or AT&T (even if such a plan does not exist yet). So, what will AT&T charge in the U.S. for an iPhone 3G data tethering service?

Wired Gadget Labs' Charlie Sorrel surveyed announced iPhone tethering plans outside of the U.S. and found a wide variety of prices and associated data caps. Some tiered plans start as low as 15 Euros for 100MB per month to as high (data-wise) as 30 British Pounds for 10GB per month. There's also a high cost 180 Euros for unlimited (5GB per month) plan.

The International Costs of iPhone Tethering (Wired.com)

Appmodo claims to have been told by an AT&T source that an iPhone tethering service plan option will be available by the end of next month (July) and priced at $55 per month.

Apple iPhone MMS Coming In July, Tethering $55

The question is if this $55 is in addition to the current $30 per month for unlimited (5GB) data or an increase of $25 from $30 to a total of $55 for both iPhone 3G data as well as the tethering service. A typical iPhone voice plus data plan without text messaging is about $70 ($40 voice + $30 data). If the tethering plan is an incremental increase to the existing data plan, this means that the total cost will rise to $95 per month ($40 voice + $55 data). This would represent a reasonably good price point compared to the current option of buying a USB 3G modem or a MiFi 3G router option with a $60 per month fee. However, if the $55 is in addition to the current $30 data plan cost, the total price rises to $125 per month ($40 voice + $85 data). At that point, it would make more sense to get a Verizon or Sprint MiFi 3G router since their networks are said to provide better 3G broadband throughput than AT&T's wireless data network.

My guess, and it is just a guess, is that AT&T has had more than enough public relations issues this summer and that they will go with the more reasonable $95 per month plan ($40 voice + $55 iPhone 3G data & tethering).