since we're all just speculating here, I'll add my educated guess :)
the original question was about tethering an iPhone to an iPad - note that the iPad runs iPhone OS (now, v3.2), NOT Mac OS X. so what you're asking is if you can tether two iPhone OS devices together. this is not supported in the current iPhone OS, i.e. you can't currently tether an iPhone to another iPhone
it would not be a simple matter of plugging them in together (no such 30pin to 30pin dock cable exists anyhow) although you could argue there's still Bluetooth possibility. but either way it would need to be supported in iPhone OS that the iPad can receive its internet connection via a 2nd tethered iPhone OS device. I'm guessing it won't be supported as such.