Stephen Bye, Sprint's Chief Technology Officer, indicated at a GigaOm Mobilize Conference in San Francisco yesterday, that Sprint will keep their data plans unlimited. Sprint apparently sees unlimited data plans as a "differentiator" from other carriers. He went on to express that not all unlimited subscribers use the same amount of data and that tiered plans contain hidden costs for customer care and support. Here's a quote from the AndroidPolice article with a few more details,
While Bye seemed to adamantly affirm that Sprint would be sticking to its guns when it came to unlimited data plans, he acknowledged industry pressure to do otherwise, emphasizing that "there's a challenge for all engineers to work on how to get the cost structure down."
That obviously leaves the door open for Sprint to change their mind in the future, as all the other carriers have done, but perhaps it will be a long time before they decide to do that. Share your thoughts in the forum.

Source: AndroidPolice