With Apple's tools, they use a separate APN for tethering. This is like having another router (you can have another APN for MMS, for instance). All your iPhone

With Apple's tools, they use a separate APN for tethering. This is like having another router (you can have another APN for MMS, for instance). All your iPhone traffic gets routed through your normal router, but the tethering traffic gets routed by your iPhone to the tethering APN. AT&T can then see from their side how much data you've put through the normal APN and how much through the tethering APN, and charge you accordingly. This is also how some of the other iPhone carriers charge you per MB, or give you a monthly xGB limit. The tethering hacks you see now just make the iPhone use the 'normal' APN for tethering traffic, which AT&T can't distinguish so easy. They might still be able to do it with deep packet analysis, but that probably costs a lot of money and might not be worth it.