Since upgrading my iPhone 4S to iOS 5.1 (from 5.0.1), the AT&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"?
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.
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+.
It means that you are connected on HSPA+. It's actually a 3G technology, and unfortunately, you probably won't experience any speed increase in downloading or surfing.
The 4G technology for the "new iPad" is LTE, and is much faster than the existing 3G band.
The 5.1 update provides a labeling change for iPhone 4S users - "3G" is now "4G". As the other answerers have noted, HSPA+ is technically considered a 4G technology. A MacWorld article states:
The 4G label in iOS 5.1 instead reflects AT&T’s perspective, namely that the carrier’s HSPA+ network (also known as Evolved High-Speed Packet Access) qualifies as 4G technology. Carriers tend to refer to both HSPA+ and LTE—the cellular network technology built into the new iPad—as 4G, though the inner workings of the technologies are vastly different.
The new iPad, however, uses LTE, a different 4G technology. There's more information about the differences between LTE and 3G in this MacWorld article.