What does the "4G" that appears on my iPhone 4S's AT&T network indicator mean? Since upgrading my iPhone 4S to iOS 5.1 (from 5.0.1), the AT&amp;T network status indicator now reads "4G" where it once read "3G". What has changed from the previous version? Does this "4G" mean the same thing as it does on the "new iPad"?
Nomenclature
The terms 2G, 3G and 4G do not refer to a specific wireless standard, but a a whole bunch of different standards:

There has been controversy on what standards should be referred to as 4G. When Apple introduced the iPhone 4S, they followed other companies in referring to HSDPA as a 4G standard. HSPA+ is the successor to HSDPA.

DC-HSPA+
Technically, HSDPA and HSPA+ are 3G wireless standards. However, HSPA+ has received technological upgrades which increased the theoretical throughput to become similar to that of LTE.
If 4G were to indicate speed rather than a wireless standard, it's legitimate to refer to the upgraded HSPA+ as 4G. However, this improved HSPA+ should be referred to as DC-HSPA+, not merely HSPA+.