At&t has a GSM network. Verizon uses CDMA. All iPhones are manufactured for GSM carriers, the world standard. Be advised that if you change to At&t the cost monthly is usually much higher. I pay about $100 a month with a 600 minute Nation Wide plan, including a $30 web fee and $15 text/MMS fee.
If your crazy enough about the iPhone then I'm sure you will pay the crazy monthly price. It's a great phone though. I've had no problems since their recent network upgrade, allowing picture and video messages. 3G has shown a lot of improvement. Reception is fine for me. It's not perfect but it beats Verizon's horrible phones with good coverage.