In most cases people who have "unlimited" plans get traffic shaping to 64KB after a couple of GB are downloaded, or they get charged extra. I wonder how this is going to pan out for those users who normally never exceed their limits, and naïvely think that upgrading the OS would fall within normal usage patterns.
Jetstar have been offering this kind of service for ages now, albeit on a rather more cumbersome piece of equipment than the swishy iPad. It's an interesting and effective way of monetising on-board entertainment services. The only problem is they don't have one for every passenger, so you might be out of luck if you're stuck at the back of the plane!