AT&T has gotten hammered over the years (well, really just since the iPhone 3G came out ...) due to a congested network. As such, it should be no surprise that AT&T can barely contain its excitement at getting its hands on T-Mobile USA's significant spectrum holdings -- especially their 1700 MHz "AWS" bands, which currently hold the carrier's 3G signal. The slide you see above (from an early morning press conference) confirms that AT&T wants to use it in order to bolster their available spectrum for the upcoming LTE rollout.
They specifically mention that HSPA+ will continue to be supported on both networks in addition to LTE. As both carriers have been marketing their HSPA+ networks as "4G" to compete with Sprint and Verizon, one would wonder what they would call LTE to differentiate it. 5G, maybe? [AT&T]