It probably does work like that, but tethering has been locked for quite a while.
Here in switzerland it is an option, more expensive than the initial contract. So rather getting an additional contract. ;-)
The more interesting question is: Will the iPhone allow tethering with the iPad although tethering with any other laptop will remain locked? This would be a pretty positive surprise for the users of the iPad and a slap into the face of the providers. Legally they couldn't do anything about it around here, when using the same allowed limit. It is just all about the software. Same goes for VOIP.