There have been rumors flying around that Sprint may release the iPhone 5 along with AT&T and Verizon this fall. Sprint currently still offers unlimited data plans and may actually offer them to iPhone 5 subscribers too.
The reason Sprint may want to keep their unlimited data plans around for the iPhone 5 are simple. More subscribers. AT&T and Verizon have already removed their unlimited data plans from their networks which leaves Sprint open to grab all of those customers that still want an iPhone with unlimited data.
Sprint stands to make quite a profit and attain a larger customer base if the iPhone 5 does launch on their network when released. However, offering unlimited data on a data hungry phone like the iPhone may cripple the Sprint network if it is not ready for the iPhone. They will need to be ready to handle a large amount of customers consuming bandwidth on their network without remorse because they have no limits.
It is a good move for Sprint, we will have to see if they do stay unlimited if and when they get the iPhone 5.