<p>As a former Cingular employee, I'd highly recommend you avoid them like the plague. While they switched back to the name "at&t", they are still using the same crappy Cingular network. The old AT&T Wireless network was sold to T-Mobile to get the original purchase past the regulators. Aside from the network quality issue, they intentionally peg their plan pricing to be the highest (on average) of the national carriers in an attempt to attract people who equate "quality" with price. The core of their subscriber base is at the Enterprise level, they don't care about their own customer service ratings, and they intentionally restrict the access of the Customer Service reps and Store personnel to prevent any "unauthorized" changes to contract or rate plan terms. The only company widely known for worse customer service is Sprint.</p>
<p>This leaves T-Mobile and Verizon. The differences between them is two-fold and your preference will determine the ideal company for you. T-Mobile uses a GSM network, like at&t, but they have made it abundantly clear that they have no plans to invest in the High Speed Data part of that network technology. Don't look for video or song downloads on their network or phones, but the GPRS data they do use is the same as at&t's which is why you can hack an iPhone to work with T-Mobile. However, GSM is also the standard in most of the world. China, Europe, Korea, Australia, and most of the Middle East all use GSM band cell phone networks. The GSM phones use SIM cards to identify and gain access to the network, allowing you to use the same handset with a different SIM card on another network provided the phone itself has been "unlocked".</p>
<p>Verizon and Sprint/Nextel use differing variations of CDMA networks. CDMA is the other technology used outside of America, but primarily in Central & South America. The precursors to Verizon went with CDMA after Analog became prone to cloning, and Verizon stayed with it as they nibbled up the regional CDMA cell companies. It doesn't use SIM cards, so phones are forever locked to the network they were designed tom be used on. While there are some CDMA networks outside the Americas, the standard is GSM. Verizon has spent the last few years investing heavily in the High Speed Data portion of their network (and to be fair, so has Sprint) and will continue to do so as technology progresses.</p>
<p>I hope this helps. If you have any questions, feel free to ask. My NDA expired a few weeks ago </p>