I don't know if T-Mobile does it, but it's certainly the way forward: http://www.saguna.net/news-events/press-releases/saguna-and-...; http://blog.advaoptical.com/is-cache-king-in-mobile-networks.

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.