Yes! Yes! Yes! Those sounds you heard moments ago are the horrified screams of your nearest AT&T towers. Why? Because iPhone OS 4.0 Beta 4 brings us a new option in the settings menu: Tethering.
PS: For those who aren't seeing the tethering option despite having the newest iPhone OS beta, just reset your network settings and that issue should be fixed. [Thanks, Zephix and Matt!]