If you already own an iPhone with a mobile data subscription, why should you have to pay for another mobile data subscription just to be able to use your iPad when on the go? Since the iPad and the iPhone use different SIM cards you can’t really move your one and only SIM card around either.

A Swedish iPhone user Jezper Söderlund decided to email Steve Jobs himself if tethering would be supported on the iPad, and he got a direct response from Steve himself:

No.
Sent from my iPhone

From Steve Jobs emails Swede that the iPad won’t tether to the iPhone (9to5mac)