Torqued over the lack of tethering support from AT&T? Taken matters into your own hands? TUAW has learned that, with the release of iPhone OS 3.1 beta 2, tethering via the IPCC hack no longer works for AT&T customers.
While iPhone OS 3.0 provides support for tethering, AT&T has yet to officially support it. As a workaround to obtain tethering (as well as MMS for some), some users have modified their carrier files (IPCC). As of 3.1 beta 2, this no longer works.
It's not that surprising that Apple would want close this loophole... but it is a little surprising that AT&T has yet to announce a firm date or rate plan for US customers to tether their iPhones.
[It should go without saying, but if you enable tethering on your 3.0 iPhone, you do so at your own risk; AT&T may slam you with a data overage bill or worse.]