There’s been a lot of details circulating about how to enable tethering in OS 3.0 when carriers don’t want you to. I got it working with a hacked carried bundle, but I just reverted to the default after I found this:
Who needs to enable iTunes debugging options when you can hit a webpage in safari on the phone, hit download and instantly enable tethering 
June 19, 2009 at 9:58 am
Update: There already carriers threatening to disconnect users who do this.