Can I use tethering with the unlimited data plan on iPhone 4?

No. Users can only tether the iPhone 4 if they are subscribed to the AT&T DataPro plan and pay an additional $20 per month.

Current iPhone owners with unlimited data plans ($30 per month) are allowed to keep these data plans with the iPhone 4 if they choose. New iPhone buyers must select between DataPlus (200 MB for $15) and DataPro (2 GB for $25).

Tethering on the iPhone 4 is a new option that costs $20 per month, and allows a notebook or other computer to be connected to the Internet using the iPhone 4 cellular data connection. Tethering is only available with the DataPro plan.