"The primary difference between the AT&T iPhone and the Verizon iPhone is the technology each uses for 3G connectivity.
AT&T employs the Universal Mobile Telecommunications System in its network. UMTS is built upon concepts from the Global System for Mobile Communications (GSM) standard, the basis for EDGE (Enhanced Data rates for GSM Evolution). Because of this shared architecture, most UMTS devices support GSM and EDGE, including the AT&T iPhone." article here
One of the other big disadvantages of CDMA is its inability to carry voice and data simultaneously. AT&T phones can browse the web while on a phone call. Verizon phones don’t have that capability, although the network is working on that feature.