Well, you can always use a proxy or equivalent method in the iPhone, although not a solution the operator and Apple permits. I do not know about the legality though since I've not heard about a case where an operator or Apple sues for using the iPhone in such a way and not paying the huge cost of official tethering.
Doing this is very easy through well documented methods and software, everything works as if it was the iPhone accessing the network, you can thus benefit from the iPhone unlimited dataplan... well in the limits of fair use.
I use this when I'm on hollidays or jump from a hotel to another for work, but I'm very cautious on the use as fair use it 500MB for my unlimited data plan. After that, max speed is caped very low.