Not to out myself as an idiot, but what the heck is 'tethering' and why would someone be excited about paying $30 per month for the privilege?
idiot proof explanation: tethering is using your cell phone like a modem for your computer. you would be able to connect the two, likely though the dock connector to USB cable (they had a usb ethernet adapter so they could probably do this the same way). then instead of the wifi you can't use cause you aren't near it, you could get your laptop online through your phones 3g service.
as for the $30, ATT has a 3g modem card and the service is at least double that. so if you could tether you could save a few bucks for the service. win-win for many