Tethering isn't against the TOS, using your wireless device as a conduit to the internet for a separate device without the appropriate plan is. So in short, tethering to use the internet on a laptop, etc is against the TOS and if they catch you, which I've seen happen a few times, they can charge you .002$/kb for the data that was used while tethering. We can see in a program called MARS whether or not the internet was used on the handheld or if the handheld was used for DUN.