Update: We've heard from two separate developers (Ari Weinstein and Jake Marsh) that they've put together quick and easy methods for enabling tethering on 3.0 iPhones (an all-in-one tool and a full user guide, respectively). Check out the links for details.
When something so appealing as the possibility of wireless Internet access anywhere you go is dangled in front of you, is it any surprise when you reflexively reach for it? After developer Steven Troughton-Smith inadvertently turned on the tethering controls on his upgraded-to-3.0 iPhone, several people made a deliberate effort to enable the feature. Posts on personal blogs and at the MacRumors forums testify to the successful quest, and there's a specific walkthrough posted and a UK-specific IPCC file kicking around.
Of course, it goes without saying that tethering your phone, in the absence of a carrier contract that permits such connectivity, is the sort of thing that may get you in a spot of trouble. Examples of such potential hazards include extremely large data bills, summary termination of your account, or hair on your palms. While jailbroken phones have been tethering for some time with few issues, past performance is no guarantee of future outcomes.