It looks like the tehering function for Starhub & M1 is hidden and you won’t be able to see this function after upgrading to OS 3.0. However, using Singtel SIM card will not have this problem. This maybe due to both telco is not in the authorize list of Apple. Now there is a workaround for that.

You can goto http://help.benm.at/sg.php via your iphone Safra , select your telco eg Starhub or M1, install this profile. After installing, perform a reboot of iPhone. You will able to see the Internet Tethering under Settings -> General > Network.