At the unveiling of the latest iPhone hardware and software at the Apple Worldwide Developers Conference in San Francisco Monday, one of the most hotly anticipated new features was tethering. But many users are likely to be disappointed by the carriers’ reaction to functionality that allows the iPhone to be used as a 3G modem.

The iPhone OS 3.0 software upgrade will be made available to iPhone users for free on June 17, and boasts hundreds of new features ranging from the mundane but useful copy and paste or landscape keyboard, through to ‘find my iPhone’, remote wipe and tethering.

Apple also unveiled a new version of the handset, dubbed the iPhone 3G S, which trumps the onboard memory and processor power of the previous version as well as boasting enhancements to the camera, and a compass.

But the operators which carry the iPhone don’t appear quite so enthusiastic about its tethering capabilities, despite the fact that many iPhone tariffs offer ‘unlimited’ data downloads.

AT&T, the exclusive iPhone carrier in the US, won’t be allowing tethering at all. While Telefonica’s UK operation, O2, will charge a princely sum for the privilege.

O2 UK users will have to buy a Bolt On to use the service, paying an extra £14.68 per month on top of their iPhone tariff to get 3GB of tethering data, or £29.36 per month for 10GB. An extra GB costs a further £4.89.

Other carriers that will allow tethering, for various prices, include Bharti Airtel, Telefonica, TIM, Optus, Orange, TeliaSonera, Maxis, Rogers, Chunghwa Telecom, Telkomsel and Telenor.