The iPad is pretty big to carry around as a cell phone... But as I said earlier, there is already tons of apps letting you do calls over VoIP like Skype. And basically, they are cheaper than a voice plan by itself. If Apple would make an iPod 3G, I would probably go that way since Skype provides an excellent service.
My voice plan is around 50$ a month for 200 minutes, caller ID and SMS plan. A Skype account is 30$ a year, unlimited minutes and SMS at a really cheap rate. We don't have skypein in Canada but there are cheap solutions. No more long distance call, no more roaming incoming calls fees...
I use Skype on my iPhone almost all the time to make outgoing calls, saving all my minutes for incoming calls. In the end, VoIP is way cheaper than a voice plan, unless you were lucky getting an unlimited voice plan.
VicoPad addict!