It appears as though AT&T is working with Apple to change the network indicator shown in the iPhone 4S status bar to display “4G” when the device is connected to their HSPA+ network, effectively marking the iPhone 4S as a 4G device.
The change (if approved) will happen in an update to iOS 5 for the iPhone 4S. While there is some debate about whether HSPA+ should be considered “4G”, AT&T is using the title merely as a marketing term to encompass both HSPA+ and LTE networks.
AT&T doesn’t address the matter of what should constitute 4G speeds, but does point out that iPhone 4S can achieve faster data speeds than a number of other handsets that are marketed as 4G devices, such as the Motorola Atrix 4G, the LG Thrill 4G, and the HTC Inspire 4G.