At the International Consumer Electronics Show in Las Vegas earlier this year Sprint announced that they would be deploying a 4G/LTE network as part of their “Network Vision” program. Network Vision is Sprint’s branding for how they are upgrading their networks for customers.
The carrier’s own 4G network should span across a big part of the country (green area) but still not cover a huge part of the mid-west where Sprint will rely on roaming agreements (blue area). With 4G LTE getting massively unveiled by Verizon and AT&T, Sprint is still behind. The carrier will only have an LTE network in its home Kansas City and in Baltimore, MD by mid 2012.
Source: @SaschaSegan