The only way to tether an iPhone is if you jailbreak it (at least on At&t) then you run into the issue of connecting it to the iPad, which would be done over bluetooth, or a custom 30-Pin to 30-Pin dock cable. This would also require a jailbroken iPad (which doesn't even exist yet) so the dev teams can't even start since you can't get an iPad, then you would need an app in the iPad to receive the signal from the iPhone, and use it for internet. Overall, it would be a pain to set up, but theoretically might be possible in the future once iPad jailbreak is possible (yet apple has taken all possible measures to prevent this) and someone has taken the time do develop an application to do this.