It's against the networks T&C in most cases but I doubt they would be able to tell, the reason is because they want you to buy their USB modem and pay for a contract. In the case of T-mobile they have higher priced plans if you want to use it to browse from your computer, again I doubt there is any way they can tell. As long as your sticking to the fair-use policy then I don't see why it is any differeny from browsing on the phone. I think you can hook up the iPhone via bluetooth to you computer, not sure if it needs to be jailbroken though.