Tell me how tethering uses more data than the iPhone. Tethering allows you to send internet data from your phone to your computer via bluetooth or USB cable. AT&T is not charged aanything when you transfer data from any device to another device using bluetooth or a USB cable, and no additional data is retrieved from the internet. AT&T just wants to receive money for a service that doesn't cost them anything to provide.
I thought it would have been common sense that a computer would chew through more data, but apparently not. Massive file downloads, P2P, file sharing, richer content webpages, more web pages, etc etc, all make your computer consume far more data that your iPhone does. I'm a heavy Internet user and my iPhone has clocked up about 650mb usage in 6 months. I can easily go through that in a day on my computer. I'm truly amazed that people expect 'unlimited' iPhone data to mean they get unlimited computer data too. Imagine how much extra data AT&T would have to serve up without receiving an extra penny for it? It's like asking Apple to drop it's prices and make less profit, and I know a lot of people around here would disagree with that.