Can an ATT iPhone be used on Verizon network?

No. AT&T uses a cell network based on GSM technology, and Verizon and Sprint use a network based on CDMA technology. The differences aren't really relative to your question, except that the two technologies are not interchangeable. Since all versions of the iPhone sold prior to the iPhone 4S contain a transceiver only capable of working on one type of network (either GSM or CDMA (only on the Verizon version of the iPhone 4)), you cannot switch between the two. The iPhone 4S uses a transceiver made by Qualcomm that will work with either type of network as needed. But that is not the case with the iPhone 4, or any earlier versions. Even with the iPhone 4S, there is an issue to consider. All AT&T iPhones, including the 4S, are locked to AT&T. If the phone finds any SIM card installed other than an AT&T SIM, it will disable the SIM, shutting off the phone and data features of the device, rendering it usable only through Wi-fi for data and voice (using an app like Skype). AT&T will not unlock the phone for you, even if you're under no contractual obligation with them.The only workarounds require "jailbreaking" (hacking, which can render it unstable or insecure) the phone or buying an unlock code from a third party (which is not guaranteed to work, and really pushes the boundaries of legality). So, when choosing your iPhone you should be careful to pick the one that uses the network type that your chosen carrier supports, or purchase an iPhone 4S that supports both. If you purchase from Apple online or at an Apple retail store, the employees will help you choose the right one. And if you purchase from a carrier (AT&T, Verizon, Sprint), they will obviously only stock the models that work on their networks. But if you buy one second-hand or off of eBay, be sure to know what carrier and network technology you will be using before making your purchase. And remember that all US carriers have their respective models locked to their service. Beginning a few days ago, Apple began selling through http://apple.com and their retail stores a fully-unlocked iPhone 4S that can work on practically any carrier. But it is available only without contract, meaning the prices start at $649 for the 16GB version (since there is no carrier paying the bulk of the cost of the phone in return for your commitment to use their service for the next two years). One additional note: If you plan to travel outside the US, and want to be able to use your iPhone on your travels, choose the one that has GSM support if possible (if you're using the AT&T network in the US), or purchase an iPhone 4S that supports both GSM and CDMA. GSM is the accepted cell technology outside the US (except for some parts of China). But remember that many iPhones are sold locked to their US carriers, which means you might have to do some creative things with the phone to get it to work on any overseas carrier. Alternatively, if you have an iPhone that supports GSM, it can be used on foreign networks through your carrier's International Roaming feature, but it is expensive.