Unlimited data plan can be WiMAX-based provided its rolled out. WiMAX is like an evolutionary step of 3G (UMTS & HS*PA). It is part of 4G. WiMAX allows faster speed than 3G, with good coverage although speeds vary greatly based on how far you are from the AP. In theory, this is a good thing for rural areas, but its also good for big cities where a large number of potential customers have 24/7 coverage in the city.

Unfortunately, telcos don't want to invest in a competitor for their rolled out 3G network, they don't want faster speed for their customers, they don't want to see customers as people who might have something interesting to share hence they prefer techniques who have limited upload speed (WiMAX is symmetric in this regard), and they certainly don't want customers to use things like VoIP. Too bad for them because sooner or later it will happen. The only question is when (and aspects like is the latency is adequate enough for general usage).