AT&T is screwing its customers again with the iPad deal. NY Times reports AT&T saying "we can handle it". Of course - particularly since they failed to live up to their promise about iPhone tethering. It's much more profitable for AT&T to prohibit iPhone tethering and then charge another $20 a month for iPad data.
But, the iPad does connect to wireless networks. If it can connect to any wireless network (just like the iPhone can) then it can connect to a wireless network set up by a jailbroken iPhone. With a jailbroken iPhone, you can set up a wireless network than anyone (or anything) can join. No different to the hardware than any other network. Since wireless is faster than 3G, there would be no difference in download speeds between a 3G iPad and the jailbroken iPhone and WiFi iPad connected through it. A jailbroken iPhone with a WiFi only iPad makes more financial sense than a non-jailbroken iPhone and an iPad with 3G and another data plan - to the tune of $240 a year. The only issue you'd have is the battery drain on the iPhone.