Most of us either use MyWi or a combo of TetherMe/iTether to tether an iPad to an iPhone. Stock ipads can only be tethered via a wifi hotspot, which isn't provided by either the commcenter hack or TetherMe. They can be tethered via Bluetooth tethering only if you JB them first and install iTether ($5 through Cydia); they can't be tethered at all via USB.
MyWi is probably the easiest solution, but the most expensive at $20. TetherMe/iTether works reasonably well most of the time, costs $7 for both, and doesn't eat the iphone's battery quite as fast, but it hasn't been a perfectly reliable combo for everyone, and it does require that you jailbreak both devices.