I am trying to decide between a Wi-Fi only or AT&T 4G new iPad. I have an iPhone 4S and my wife has an iPhone 4; two data plans seem like enough to me! My question relates to tethering. When tethered, does the iPad send the signal to the iPhone, or does the iPhone send the signal to the iPad? If the iPhone sends the signal, can I upgrade to the $50 data pro plan on a month-by-month basis, or would I have to buy a long-term plan that includes tethering every month? Thanks.