You can jailbreak your iPhone and than purchase MyWi, a tethering app, which will let you share the iPhone's 3G connection via Wifi with other wifi-capable devices. So you would start MyWi on the iPhone, turn on Wifi-tethering and than just join the iPhone's Wifi.
MyWi, or any other tethering app for that matter, is not available from the AppStore, so a jailbroken phone is a requirement for this to work.