Can I use tethering to let an iPad use an iPhone's 3G? Given an iPad that doesn't have 3G but only wifi, can I make this use the 3G connection of an iPhone 4? 

Does this require a cable between the devices, or does it work over (bluetooth|wifi?) ?
I don't believe you can do this with the stock phone, however if you decide to jailbreakyour iPhone, you could use an application like MyWi to share your iPhone's 3G connection over WiFi