Last October, an internal AT&T talking point claimed that the carrier was working with Apple to have the iPhone 4S status bar display a "4G" indicator when the device is connected to an HSPA+ network.
With today's release of iOS 5.1, that change now appears to have gone into effect as shown in a photo posted to Twitter by @ichadman.
While short of the data speeds seen with LTE networks now rolling out, AT&T and other carriers have also referred to their HSPA+ networks using 4G as a marketing terms to distinguish the standard's faster performance relative to older 3G standards.
Article Link: iOS 5.1 Adds '4G' Indicator to iPhone 4S Status Bar for AT&T HSPA+ Coverage