Some people have already bought mobile hotspots for their car. For example, one popular unit allows you to use Verizon EVDO 3G and share it via WiFi. Your iPhone carpool buddies will love it.
I proposed a while back that perhaps something similar could be set up using a 3G Windows Mobile or Linux handheld. Set up a webserver on it with your favorite static iPhone web Applets. Point iPhone Safari at it over WiFi and off you go.