I've only read of problems with people using iphone plans on the ipad, not the other way around. Why does this matter? Well, because they want you to pay for ipad plans and don't need to worry about the ipad plans being used on the iphone since they only sell the iphones with plans (no unlocked iphones in the US right now). In other words, they just want to make sure you have two plans if you have an iphone and an ipad. Notice though, you can use your ipad plan in any device that can take a micro-sim and change the APN. I haven't heard of anyone having their ipad plans canceled or blocked.
Now, if Apple starts selling unlocked iphones in the US, things might chance.