All iPhone 4Ses are equal, but some iPhone 4Ses are more equal than others.
This is the Orwellian approach that AT&T
The iPhone 4S models will actually be the same, but the difference lies in AT&T's network architecture. The iPhone 4S was built to support a technology called high-speed downlink packet access, or HSDPA, which is related to GSM networks like AT&T's and T-Mobile's, while Sprint and Big Red use a different technology called evolution-data optimized, or EVDO, with their respective CDMA networks.
The 4S supports up to 14.4 Mbps download speeds with HSDPA, but Apple wasn't quick to point out that those theoretical maximum speeds won't apply to Sprint and Verizon at all. This means that two out of three of the domestic carriers won't be able to take advantage of the higher theoretical speeds and will be relegated to the previous generation's maximum speeds.
This all sounds fine and dandy for AT&T, in theory. The problem is that in reality, AT&T's HSDPA network capabilities are nowhere near the theoretical maximum of 14.4 Mbps. AT&T's developer site provides an explanation of its various wireless technologies, and here is an excerpt from its description of HSDPA:
HSDPA -- an enhanced protocol in the HSPA family -- is the highest-performance cellular-data technology ever deployed. Its peak theoretical rate is 14.4 megabits per second (Mbps). AT&T has engineered its network so that most users' experience typical downlink throughput rates of 700 kilobits per second (Kbps) to 1.7 Mbps, with bursts over 1 Mbps. Typical uplink rates are 500 Kbps to 1.2 Mbps.
Source: AT&T Developer Program site.
The site is current as of Sept. 2, so this description is fairly recent. You'll notice that in real-world performance, the company concedes that users typically get up to only about 1.7 Mbps. This doesn't even come close to the previous iPhone 4's theoretical maximum of 7.2 Mbps, much less the iPhone 4S's 14.4 Mbps limit.
There is one wireless carrier that has a beefier 14.4 Mbps HSDPA network: T-Mobile. Even though AT&T has been trying to scoop up the smaller carrier, T-Mobile's network operates on different frequencies than the iPhone 4S supports, so it wouldn't be of much use to the device.
A network by any other name would download just as fast
Both AT&T and T-Mobile have been quick to advertise each of their networks as "4G." This is mostly a marketing gimmick bound to confuse the average consumer. The International Telecommunications Union, one of many international organizations that try to call the shots, has decided that the only technologies that are truly "4G" are LTE and WiMAX, although the semantic debate continues to rage on. The top three carriers have opted for LTE, while Sprint is practically abandoning Clearwire
AT&T and T-Mobile have continued to heavily market their networks as 4G. Apple has shied away from participating in the debate but instead focuses on the fact that the iPhone 4S does support speeds on par with other Google
Guilty by association
Taking the marketing shenanigans a step further, AT&T has confirmed that it is "working together" with Apple on adding a 4G indicator in the iPhone 4S status bar, which would be exclusive to its network. The possible addition would be done by an iOS update directly from Apple. No decisions have been made, but it's notable that Apple is even considering this, since it has long refused to give in to carrier demands. If Apple goes along with AT&T's request, it would be implicitly siding with AT&T and its 4G marketing.
Regardless of whether Apple acquiesces to the plot, don't expect AT&T's marketing department to let up. The commercials haven't started yet, but you can bet your first-born child that they'll go something like this: "AT&T, the only network to get the all-new iPhone 4S with blazing fast 4G speeds!" In fact, the carrier has already put out a press release to that effect, proclaiming, "Only AT&T's Network Lets Your iPhone 4S Download Twice as Fast."
The real McCoy
Don't buy into cheap marketing gimmicks. All internet service providers, wireless or otherwise, have a bad habit of advertising maximum theoretical download speeds and trying to absolve themselves with fine-print disclosures. The reality is that none of the networks actually get close to those speed limits. I'll be waiting for next year's iPhone, as I'm fully expecting it to come with real 4G in the form of LTE, and it might even include near-field communications.