I'm not certain that what I'm about to write is correct, but I've read that the no data cap is only for 4G phones, so if you happen to have a 3G smartphone you're going to pay the extra $10/mo and still have that 5GB "soft" cap.
Like I said, that may be completely wrong, but this is what I've read. I'm still very skeptical that Sprint is actually not going to cap data for 4G phones, considering the reasons why everybody else does it (people using their phones as wire line replacements and taking up tens or hundreds of GB/month downloading Blu-Ray torrents, etc.). My guess is this will end one of two ways: either Sprint will create a soft cap, or they'll go the VZW route and throttle people that use the top n% of data.
Followup Edit: Sprint's rates are indeed among the lowest (I think t-mo is pretty close), but with this new fee they are creeping dangerously close to carrier-ambivalence for me. If another carrier had substantially better service or a device that was exactly what I wanted, at this point the price difference isn't as big a deal and I'd probably switch.