No.
Not as of firmware 1.1.4. Though recent updates saw an extension of the iPhone's VPN protocol support, LEAP/PEAP WiFi support was absent.
LEAP/PEAP support, along with many other features, is rumored to be coming with the iPhone 2.0 release but this has not yet been confirmed.
UPDATE: Internet tethering on the iPhone 3GS and iPhone 4 is called the Personal Hotspot. The feature must be activated with your carrier at an additional monthly cost. An iPhone 4 or iPhone 3GS can share their cellular data connections with other devices via Bluetooth or USB. The iPhone 4 is also capable of creating a Wi-Fi hotspot for other devices to connect.
UPDATE: The iPhone 3G offers Internet tethering via Bluetooth or USB. This feature must be activated with AT&T at an additional monthly cost.
