From what I understand, you can use apps like EZ Tether or PDA Net without having to root, but that requires tethering via a USB cord. For tethering through enabling WiFi hotspot on your phone without a tethering plan, you would need to root. Without rooting, enabling the WiFi hotspot on your phone would be disabled and require adding a tethering plan before it would work.
FWIW, the carriers are going after people who tether without paying for a tethering plan. AT&T is especially agressive at it. They started with iPhone users, but there are more and more reports of them going after Android users as well. Basically, if they catch you tethering, they send you a warning to stop or they will automatically add a tethering plan to your account. The problem is if you are grandfathered into an unlimited data plan and they switch you, they will switch you to the $45 4GB tiered tethering plan, so basically you lose unlimited data.