According to an article over on Cnet, the release of iPhone 3.1 firmware is blocking a tethering hack. Apparently once you have updated to iPhone 3.1 and you navigate to Settings app — General — Network, you will find that tethering is now gone from the menu.
Apparently the loss of the tethering hack isn’t affecting everyone as varying versions of the AT&T carrier file looks to be the reason for this.
iPhone developers so far haven’t been able to come up with a way to downgrade or even re-enable tethering, so for the moment it looks like if you have lost tethering its gone until a new hack can be found.