The DeviceKnit blog has put together a handy little how-to in order to help matters. It will require that you have tethering on your plan, but I think that’s a worthwhile $15 for FaceTime away from WiFi.
I really don’t have much to add to what DeviceKnit is already saying, so all credit for this one is going to them:
Turn on tethering in Settings > General > Network > Internet Tethering on your iPhone 4 and connect it via USB. You’ll see a new “iPhone USB” connection option pop up in the Network preference pane in System Preferences.
Go to the Sharing preference pane, and turn on Internet Sharing to share your USB connection over WiFi/AirPort
Now, connect to the network you just created with your iPhone, and you can FaceTime all you want. Hopefully when there’s a jailbreak for the iPhone 4 the laptop and tethering won’t be needed anymore. Enjoy!