The only obvious clue that you're holding one carrier's iPhone vs. the other is the Verizon or AT&T name that appears in the upper left corner of the screen. Slits for the wireless antennas that communicate with the various networks reside in different locations on the top and side of the respective devices.
Buttons on the Verizon device are in a slightly different position as well, which means that optional protective bumpers that Apple temporarily gave away in the wake of the antenna controversy on AT&T don't fit the new iPhone. Apple will sell new universal $29 bumpers that fit either iPhone 4.
Whatever pent-up demand there is for the Verizon iPhone, I wouldn't advise AT&T iPhone owners to bail immediately (if at all), unless your coverage is untenable and unless you're willing to fork over an early-termination fee of $325, less $10 for every month that you are into your two-year contract. In my experience, using a number of phones through the years, Verizon often outperforms AT&T, but not everywhere. In an area of South Beach, Fla., that I frequently visit, for example, AT&T has superior coverage.
And, all things being equal, the AT&T GSM network is actually faster than Verizon's CDMA network, though a number of factors contribute to network speeds and rarely will you experience anything close to the theoretical maximum. My own speed tests in the New York area were inconclusive.
For business travelers, it's also worth noting that the GSM wireless standard is more broadly accepted abroad through roaming agreements. AT&T service is available in more than 220 countries around the world. Verizon's CDMA roaming service is available in about 40 countries, with most of Europe off-limits to the Verizon iPhone.
Another key difference: On AT&T's network, you can gab on the phone at the same time you're pulling in data or surfing the Web. So you can check e-mail, trace a map route or search the Internet for a restaurant where you and your caller can meet up.
If you try to surf on the Verizon iPhone while on a call, a pop-up message warns, "Cellular data connections are not available during this call." A Verizon iPhone user can continue to surf and talk if using Wi-Fi.
Though much is made of this simultaneous voice/data limitation, it probably won't be a deal-breaker for most Verizon-hungry consumers. There aren't that many occasions where you want to do both things.
If you are using a navigation application when you receive a call, the navigation pauses while you take the call and resumes when you hang up. And when on a call, you can send, receive and read text messages. You can use the iPhone for other purposes while a call is in progress, such as going through your contacts list to pull a phone number.
Verizon iPhone 4 owners, meanwhile, can exploit a personal hot spot feature, not currently available to their AT&T counterparts, that lets them share an iPhone 4 Internet connection with up to five Wi-Fi-ready devices. (AT&T says it's adding a hot spot app, but it hasn't said how many devices can connect. The price will be $45 for 4 GB, when bundled with a data plan.)
To help folks keep track, an indicator appears on the phone's screen to let you know how many Wi-Fi devices are connected. If no devices are connected for 1½ minutes, the hot spot turns itself off to preserve battery life. The feature costs $20 a month for up to 2 GB of data, and $20 for each additional GB. You cannot use the hot spot while on a phone call.