FYI, the only carrier without any semblance of a next gen network is AT&T.
Verizon has LTE, considerably faster than their 3G. Sprint has their WiMAX, considerably faster than 3G. T-Mobile has been building the infrastructure to support HSPA+ on their GSM network for two years, making it just as fast as Wimax and LTE in a lot of cases. AT&T has just been riding their iPhone gravy train for years, without making any investments in next gen infrastructure or technologies. Heck, their LTE plans are to buy T-Mobile, kill their 3G/HSPA+, and use the spectrum for their LTE network.
since 2009 when i bought iphones AT&T has gone from UMTS to HSPA to HSUPA and now HSPA+