The good news is that Sprint has promised that WiMax will always be unlimited. And there's a technical reason to back up their promise. Because WiMax operates at a higher band than Verizon's LTE 4G, it offers more bandwidth by its physical wavelength properties. That means a ton of people can use the infrastructure without degrading the performance.
However, the drawback to WiMax is that because it operates at a high band, the waves don't travel as far as a lower band. This means that to get a good signal, you need to be near the towers. That means Sprint would need a ton of towers to bring WiMax to everyone.
This is the reason why 4G coverage is currently very spotty. Getting Wimax to everyone is not practical. Which is why Sprint is toying with the idea of simultaneously supporting LTE as well. This means that the majority of Sprint customers will eventually have good 4G speeds and coverage; however, LTE will likely be capped because of it's reduced bandwidth capability.
So, long term, I think Sprint is the only carrier with a shot at offering true unlimited plans. The catch is that it only applies for WiMax. If you live in a populated area (high population density), you're in luck. If you're somewhat rural, you may not get unlimited data, but at least you may still have 4G (via LTE) as an option.
A lot could change though. Smartphones use the vast majority of the bandwidth, and smartphones only have 20% penetration of the overall mobile phone market. Adoption of smartphones is skyrocketing, so pretty soon, the carriers have to figure out some way to handle all that capacity. And the consumers have pretty much spoken about not wanting caps. So... carriers are going to have to invest more money. And that means the cost of our plans are going to go up in response. That's exactly the reason why Sprint is now enforcing that $10 charge on all smartphones now.
Pick your poison