Having jailbroken my iPhone a few months back. One of the most invaluable apps is MyWi. MyWi creates a WiFi access point for your laptop (and the laptop of friends) to connect to. No need to create an ad-hoc network, nada.
Performance is pretty decent.
NOTE: MiWi requires a jailbroken iPhone. See my prior post on blackra1n for information on quickly and easily jailbreaking your iPhone.