On the one hand, my usage is around 500 MB/mo right now (mostly thanks to trivial access to institutional wifi at home, work, and school), so this would save me $5 and enable tethering. On the other hand, the concept of being tempted down from unlimited usage is somewhat offensive. On the other other hand, over-the-air bandwidth is a location-limited resource and switching from unlimited might make me a better netizen. On the other other other hand, I have no idea how much bandwidth I'd start using if I was running my WiFi iPad through my iPhone?
I think I'd want to see some kind of "grace period", that is, the first month you ever go over your current plan's limit you have the opportunity to switch to a larger plan without overage fees, but it would never fly.