In theory this seems like a reasonable network management practice except for at least two factors -
1. Roughly speaking, what amount of data would put me into the 5% user range?
2. Will you throttle at time specific intervals or just in general? Throttling my speed during peak traffic times seems reasonable, but doing it at 3am seems pointless and doesn't increase anyone's QoS.
It's also a sign in my opinion that Verizon is not nearly as confident over the medium-long term about their network's ability to cope with new iPhone users as they pretend to be if they're already taking these kinds of steps before the phone has even reached consumers hands.