I wonder if the contract allows US Cellular to sub-lease the naming rights.
They probably wouldn't want to grant the naming rights to Sprint, as Sprint will still be a competitor of theirs, just not in the Chicago market. They wouldn't want the Sprint mentions when the Sox are on ESPN, MLB, or WGN America, as those broadcasts would reach markets where Sprint and US Cellular still compete.
I'd guess that we'd be seeing a non-cell phone name on our stadium within the next couple of years. Any other national businesses that have a great relationship with the Sox?