It's not a bad idea, and we're likely to see more carriers jump on this particular bandwagon as the iPad gets closer to release. What I would love to see from carriers is a way to extend existing service for iPhone customers to those who want an iPad plus iPhone. AT&T already gets close to $100 a month from me for basic cell service, plus iPhone data and a text messaging plan. I wouldn't mind paying an additional $5-10 a month for my iPhone contract to cover an iPad as well.
The thought of having to pay another $15 or $30 a month for extra 3G service to an iPad, on top of the charges already being paid for an iPhone, leaves a bitter taste in my mouth and if the decision is made to have an iPad join my family, I'm most likely sticking with Wi-Fi only.