It has been clear since the first quarter of 2010 that Sprint had other plans for use of the spectrum that currently supports the iDEN network for Nextel. See http://community.sprint.com/baw/message/175739. Later in 2010 Sprint began to talk about putting some CDMA (3G) services into the vacated iDEN spectrum. See http://www.bgr.com/2010/10/27/dan-hesse-sprint-will-eventually-shutdown-iden-network/.
Sprint has been talking about adding Long Term Evolution support as well, but it is not clear whether it will use some or all of the vacated iDEN spectrum for LTE. The original grant of iDEN frequencies were in the land mobile area, and typically the amount of spectrum is not overly generous, as the original application was not bandwidth intensive. See http://www.ntia.doc.gov/osmhome/allochrt.pdf.
The iDEN network uses the 806 MHz to 824 MHz and 851 MHz to 869 MHz bands, meaning there is 18 MHz in total available for outbound and 18 MHz available for inbound communications. LTE tends to operate in either 10 MHz or 20 MHz channels, and the wider the channel the faster the speed. That means Sprint could, in principle, replace iDEN with LTE in the 800 MHz spectrum once it decommissions the older iDEN network.
Of course, there is business logic for pushing low-bandwidth CDMA traffic into the iDEN bands, especially voice and mesaging, since these apps don’t use much bandwidth. That would free up more bandwidth on the 3G network. On the other hand, there also is logic to creating a new LTE network using all of the 18 MHz spectrum, which would allow building of a very-fast LTE network.
Sprint already is replacing its current infrastructure, supporting three different networks with three sets of radio gear, into a single set of radios that can support all the frequencies Sprint uses, including the 800-MHz iDEN, 1.9 GHz CDMA and 2.5 GHz WiMAX networks. See http://newsroom.sprint.com/press_kits.cfm?presskit_id=19.
On the other hand, there also is the possibility that the 2.5 GHz band, which has spare bandwidth not in commercial use, also could be used to support LTE. That would require a matching decision by Clearwire to adopt LTE in some of the 2.5 GHz band, but is not inconceivable, either.
The advantage of putting low-bandwidth services such as voice into the vacated 800-MHz spectrum is the superior in-building reception.