The distinction between USB tethering and wifi tethering (aka hotspot) is becoming more clear as users are getting out there and testing on those demo models and I/O gift phones. In short, it sounds like USB tethering is already built in - or at worst possible via PDAnet - so you can connect a single laptop to the Evo by USB and use the Evo as a modem. The wifi hotspot function which provides up to 8 wireless devices with internet access through the Evo is the function for which Sprint is charging $30/month.
Additionally, rooting the phone won't necessarily get you around the multi-device hotspot fee, as Sprint can control that connection (evidently). This is a gray area as I don't think anybody has actually tried it, and the rooting process is still privately held until after launch.