Apple’s Scott Forstall mentioned that Gizmodo has posted a tutorial
on how to enable the hidden 3G data tethering in the developer builds of iPhone OS 3.0.
It's as simple as installing a 36KB file called "ATT_US.ipcc" on the iPhone (running 3.0) then flipping a new "Internet Tethering" switch in Settings > General > Network.
The problem is the fine print. Gizmodo warns that tethering over Bluetooth (rather than USB) only works with older hardware and not with the unibody MacBooks, for example.
A Gizmodo reader warns of other potential problems with the hack including issues with MMS, voicemail and billing. AT&T could bill you retroactively for usage.