You know what would be cool? And what would make me buy a 3G iPhone instantly?
A mode where the phone acts as a WiFi access point, using the 3G connection for Internet access.
So you basically turn on that mode, and you can access the net trivially through your phone from any WiFi enabled computer.
THAT would be a killer app for the iPhone. Imagine Internet access for your computer anywhere in 3G coverage area. No dealing with bluetooth or cables, just use WiFi.
Since the iPhone already has a WiFi chipset, this would be trivial to implement, and incredibly useful to boot.
Come on, Apple. Do it.