It will stick around
Mark my words, Sprint has every incentive in the world to keep its unlimited data. AT&T capped the data last year because they were still the only carrier with iPhone, Verizon did the cheap trick of getting people to switch and then cap data because they are America's biggest network.
Sprint has always been the smallest and the one to stick it up to big carriers. As I mentioned in an earlier post, Sprint was the first to offer nights starting at 7, unlimited web on Windows and Palm phones for only $15 (same plan used to cost $30 on Verizon), and free calls to any network mobile.
Coming as the THIRD carrier of the iPhone, Sprint has every incentive to keep unlimited data plans.
I wouldn't be surprised if later down the line they keep the unlimited data AND offer cheaper option capped data, just to stick it up to AT&T and Verizon.