There has been much discussion and the search for the Netshare app, which was originally in the iTunes store. However, now, in the US, Steve has been discussing with ATT to try to get tethering done for the iPhone.
With other phones, you would have to pay 30USD a month, but not in this case. If not, other developers like the one that made Netshare, would keep releasing Apps, and making ATT lose money.
You would still have to say that the plans in Singapore for the iPhone are pretty outrageous as compared to the US.