The IPCC tethering hack used to allow use of tethering even if your carrier did not support it has now been stopped in 
iPhone OS 3.1 Beta 2. It’s not a surprise that Apple would fix this as not all carries have sorted out price plans for tethering.
If your still on iPhone OS 3.0 and don’t upgrade to 3.1 when it is officially released then the loophole will still be there and you can still use it at your own risk as if your carrier releases your using tethering without paying for it then can charge you at full data rates.
Sam's Blog 