If your carrier doesn’t support tethering your iPhone, to use it as a 3G wireless modem, help may be at hand Tether Me will generate a custom APN file that you can add to your iPhone to give you a tethering profile. I feel that I must repeat the disclaimer on the Tether me! website
Disclaimer: While this enables tethering, I don’t know for sure if your carrier will be able to detect that you are using it. I assume not, however I may be wrong. Keep an eye on your bill