Not "supposedly" unlimited. It IS unlimited. You won't get charged any overage fees. You can use the web 24-7 for the whole month, if you were so inclined and never slept.
It's unlimited for what your phone can consume in a month. It does not, however, include tethering (using your phone as a high-speed modem for your computer or laptop).
Has anyone actually been charged for tethering yet?