Mobile links are in fact quite often backhaul limited. Especially as you make cell sectors smaller (and in particular with T-Mo, which uses higher frequencies and needs to deploy smaller cells), getting data off the cell sites can become a significant bottleneck: http://www.pcworld.com/article/251838/analyst_mobile_network...
Not yet, but with SD cards now at 512GB capacity, its not out of the realm of possibility to do caching of popular content at-scale across your tower infrastructure.
