Actually Sprint is using two different LTE strategies. They partnered with Lighsuared for LTE initially, but they might not even get their network deployed since the FCC is complaining about how their LTE if deployed would mess up GPS in aircrafts. I believe because of this announcement, Sprint is rolling out its own LTE network which they say will go live in Mid-2012 or probably August 2012.
Clearwire is also deploying LTE but dont expect it to ever become a reality if they can not get any funding. I have the epic and decided that I would not upgrade devices unless it was a Nexus S. I expect LTE to go live in July or August with LTE devices from Samsung (Galaxy SIII), HTC (Evo Line), and Google (Nexus Line) coming in time for the fall 2012.
Also Apple is probably going to deploy an iPhone 5 with LTE with an announcement coming in August 2012 and pre-orders/sales coming in the beginning of September. Im not trying to support apple, but if that is the case, I am sure Sprint will be ready to sell iPhone LTE devices with at least a working LTE network.
So I figure LTE will be live around August or September 2012. If you can hold out a year do it.