iPhone developers that or officially registered in Apple's Developer Program have been seeded the beta 2 version of the iPhone OS 3.1 firmware. While Apple hasn't announced any official "new features" that the beta 2 version includes, one thing is certain: it kills the tethering hack.
Apple announced that tethering will be possible in the new iPhone OS 3.0, and while it indeed is, American customers cannot tether due to AT&T's inability to prepare the groundwork on time. American users must wait for AT&T to get everything together before they can start tethering and using MMS.
However, someone found a hack which allows iPhone users to tether, despite their service provider not allowing them to. This hack became extremely popular and spread all over the web like wildfire. If you haven't tried it yet, find the step-by-step instructions here:
The new beta release disables this specific hack from working. This is interesting for two reasons. For one, we now know not to update to 3.1 when it comes out if you really need tethering. I'll repeat that:
Do not upgrade to iPhone OS 3.1 when it comes out if you need internet tethering.
Second, it's interesting to see how Apple pays attention to everything that goes on in the "underground" iPhone world. Looks like AT&T really doesn't want anyone tethering if they went as far as to push Apple to block the hack in a beta upgrade.