[HTC Vivid, Raider, Velocity] bjg222:
OK, so it's come up in several threads, so I thought I might try and simplify things and answer some people's questions with a bit of a primer on GSM, HSPA, LTE, 3G, 4G, etc! So here goes! (And special thanks to ClippinWings, I liked the way you formatted this list, so I'm gonna borrow it! ;))

GSM Data Technologies:

So in order from slowest to fastest, the GSM data technologies are:

Legend:

Technology | Vivid Taskbar Icon | Standard Android Icon | Actual GSM Category

GSM | G | G | 2G

EDGE | E | E | 2G

UMTS | 4G | 3G | 3G

HSPA | 4G | H | 3G

HSPA+ | 4G | H+ | 3G*

LTE | 4GLTE | ? | 4G*

LTE-Advanced | N/A | ? | 4G

*3G/4G status here's disputed, I'll get to that later

The Vivid, displays G & E, then displays 4G for 3G, H & H+, then 4GLTE for LTE. So no matter what 3G network your on, it'll show 4G (because AT&T thinks H+ counts as 4G, more on this later), then when you're on an actual 4G network, it'll show 4GLTE.

Difference between Verizon & AT&T LTE phone radios:

AT&T's network is completely GSM based for 2G, 3G & 4G, whereas Verizon's 2G & 3G network is CDMA based. Therefore, one radio will work for everything in an AT&T LTE phone. This is different from Verizon, where the two different technologies necessitate two different radios: one for CDMA, one for LTE. This will most likely continue to be the way it works: Verizon (and eventually Sprint) phones will require two radios for two technologies, AT&T/T-Mo phones will only require one.

LTE networks are completely data focused and don't yet implement voice protocols. When they do, it will be VoIP based, so it'll just be more data (This will be called VoLTE for Voice over LTE).

This leads to one consequence, however, concerning simultaneous data & voice. Since this isn't implemented yet, phones must revert to the 2G/3G network to make voice calls. On Verizon, since they're two different radios, you can continue to use LTE data while using CDMA for voice (A new feature for Verizon, as CDMA networks don't handle simultaneous voice & data). On AT&T, they're the same radio, so dropping down to 3G drops the LTE connection. You can still use simultaneous voice & data (as you can on just about any GSM based network), but you just don't get LTE speeds.

Is HSPA+ Really 4G?

Easy answer? There is none. By ITU (the body that decides these things) original standards, 4G was any technology that could provide 100 Mbps downlink. By these standards, not even LTE qualifies. Only LTE-Advanced, the as of yet unimplemented successor to LTE, qualifies (Verizon, Sprint, & AT&T have all committed to LTE-A deployments starting in 2013). However, LTE being a new protocol that uses new frequency bands, it's natural to call it 4G. HSPA+ is an enhancement to 3G technology to provide a short term solution until LTE deployments are complete (mostly through enhancements to fiberoptic backhaul), and is very often called 3.9G (FYI, HSPA is commonly referred to as 3.5G). However, T-Mobile, since they didn't have an actual 4G deployment plan, started calling their HSPA+ deployment 4G. Especially since it gave speeds just as fast (or faster) than Verizon's LTE network, this caught on, and AT&T was forced to do the same to keep up. With HSPA+ squarely being called 4G in advertising, the ITU amended it's definition of 4G. It said that LTE, being fundamentally new, was clearly 4G. It also made a statement that, while not officially calling HSPA+ 4G, justified T-Mobile's & AT&T's use of the term. And since HSPA & HSPA+ are basically the same technology, the Vivid & Skyrocket just show 4G for it all!

I hope this helps clear up some confusion. I made every effort to ensure all of this info is backed up by research, but if I got something wrong, please let me know! Or if there's anything you think I missed that should be included here, please let me know that, too!