TetherMe – $2 in Cydia.
TetherMe dynamically enables native tethering for iOS 3.x and iOS 4.x+ devices. This should work on any carrier without the need for customized bundles or preferences.
TetherMe also bypasses signature checking and allows you to edit your carrier bundle manually if you so desire. This is required to enable MMS functionality on T-Mobile. TetherMe does not enable the loading of APN configuration from .mobileconfig profiles at this time.