try it again, I've got 3.1 and its working great - just make sure to follow everything exactly step by step - you might need to reboot too
if your iphone is already jail broken, I'm not sure it will work with that (but it should)
It Figures...When Apple released its highly anticipated iPhone OS 3.1 firmware update for the iPhone and iPod Touch, some users' worries were confirmed: the tethering hack we blogged about in June no longer works.
After upgrading, if you navigate on your iPhone to the Settings app, select General, then Network, you will see that the menu item for tethering has vanished.
Developers have not yet been able to find a way to downgrade or re-enable the tethering hack. If you or someone you know has had a different experience, we would like to hear about it in the comments.
Updated on 9/10/2009 at 10:15 AM PDT: According to user comments below, tethering remains intact for some, but not for others. Varying versions of the AT&T carrier file seem to be responsible for the discrepancy. We'll be seeking comment today from AT&T and Apple regarding these carrier files.