The writing was already on the wall as Sprint began "aligning their business practices with [their] competition." Many may refuse to admit it, but those who have been with Sprint at least two years had to know/knew data caps were coming. Hesse said it himself; it was just that he nor anyone else knew when it would happen.
That being said, I can't see Sprint imposing data caps at the consumer level until after LTE is fully deployed. The inconsistent level of service won't allow it.
As it is, their small LTE footprint has consumers chomping at the bit for better 3G speeds where not covered by LTE. Add to that the $10/mo "premium" fee and nominally better pricing than Verizon (at least in the DC metro area), dwindling list of features that separate them from the VZN/ATT, etc, Sprint may as well toss in the towel as subscribers will flock to [perceptually] better alternatives.