Irish programmer Steven Troughton-Smith discovered the phone-as-modem feature's presence in the beta by accident earlier on Wednesday after uploading a custom Carrier Settings file to overwrite the defaults for O2, his local provider.
While understandably spotty due to its beta state -- the phone initially hard-locked on making the computer connection -- tethering has successfully given Troughton's Mac 3G data access over USB as though it were another network adapter. The feature is exposed in the iPhone's network settings as a simple slider and describes the feature as only requiring a USB connection or a Bluetooth pairing, though the latter hasn't yet been tested.
When connected, an iPhone shows a blue "Internet tethering" status bar below the clock to remind the owner that the tethering link is still active and consuming data, much as existing iPhone firmware will display a green bar to indicate an ongoing phone call.
The exposed feature, noticed by MacRumors on Wednesday, confirms remarks by senior iPhone software VP Scott Forstall that tethering is present in iPhone 3.0 itself but would depend on commercial and technical support from individual cellphone carriers before it could become a simple option for subscribers.
At this stage, it's not possible to tell which if any carriers might already be enabled for iPhone tethering, though only AT&T has set out plans for the data modem link to become an option sometime in the future. Some providers, like Rogers Wireless in Canada, already factor tethering into their plans for most smartphones and wouldn't have any pricing limitations as a result.