As it stands, the iPhone can only share a data connection via USB or Bluetooth. The iPad doesn't support Bluetooth data sharing (because it doesn't support the right Bluetooth profile). Neither supports Wi-Fi data sharing. According to jailbreakers, it is possible for the two devices to share a data connection via Wi-Fi with the right applications, but they aren't official apps sanctioned by Apple. You'd have to jailbreak both devices to use them.
Right now, if you were hoping to skate around AT&T's new data plans by tethering your iPhone to your iPad, it looks as if there's no legit way to do it. If AT&T and Apple were smart, they'd figure out a way to make it happen.
Speaking of data plans...
The iPad is in short supply right now. Apple is having a hard time keeping it in stock. Customers hoping to score an iPad 3G and snag one of AT&T's unlimited data plans for $30 month would appear to be out of luck, since that plan will no longer be available after June 6. AT&T is looking into a solution, claims one insider.
"We are looking into this situation and how we can accommodate these customers," a source told The New York Times The temporary fix that's being considered? Allowing potential iPad customers to purchase the unlimited 3G plan even if they don't yet have the iPad itself.
If AT&T doesn't change something quick, Monday's changes spell doom for future iPad 3G owners. They will be stuck with AT&T's $15 250MB option, or the $25 2GB option.
Updated. Apparently, AT&T has in fact decided to give the $30 unlimited 3G plan to people who order the iPad before June 7. So, if you want an iPad 3G with the $30 unlimited data plan, order it before June 7, and AT&T will honor the plan.