O que você quer é chamado de " tethering ".
Aqui você pode encontrar a posição oficial da Apple: | Da mesma página (quando você clica em 'Como usar o compartilhamento de internet'):
Using iPhone as a Modem
You can use iPhone 3G or later as a modem to connect, or tether, your computer to the Internet. You can connect iPhone to your computer using the Dock Connector to USB Cable, or via Bluetooth.
NOTE: Additional fees may apply. Contact your carrier for more information.
Tethering works over the cellular data network. You can’t share a Wi-Fi connection to the Internet. If you have a 3G connection, you can make and receive phone calls while tethering.
NOTE: To use iPhone as a modem with a Mac computer, it must be running Mac OS X version 10.5.7 or later.
Set up a tethering connection:
In Settings, choose General > Network > Internet Tethering.
Slide the Internet Tethering switch to On.
Connect iPhone to your computer:
USB: Connect your computer to iPhone, using the Dock Connector to USB Cable. In your computer’s Network services settings, choose iPhone.
On a Mac, a pop-up window appears the first time you connect, saying “A new network interface has been detected.” Click Network Preferences, configure the network settings for iPhone, then click Apply. On a PC, use the Network Control Panel to configure the iPhone connection.
Bluetooth: On iPhone, choose Settings > General > Bluetooth and turn on Bluetooth. Then refer to the documentation that came with your computer system software to pair and connect iPhone with your computer.
When you’re connected, a blue band appears at the top of the screen. Tethering remains on when you connect with USB, even when you aren’t actively using the Internet connection.
Monitor your cellular data network usage:
In Settings, choose General > Usage.
Existem algumas outras formas explicadas na internet como fazer isso quando você sabe qual termo usar procure por; -)