I have broadband on my AT&T Tilt, but it costs extra to get a plan to use it to tether a laptop or PC. Since my broadband is unlimited, why should it matter if I hook up a laptop with a USB cable? Will it work without upgrading my plan? Is there a work-around?

A "work around" would be a violation of your contract with AT&T, and we do not provide information to help with such violations. As for the reason, bandwidth is a commodity that has a limitless demand and telecommunications companies are having a very hard time justifying their "Unlimited" broadband plans as consumers become more accustomed to living their life on the internet.