Here's a clear and informative breakdown of GSM/EDGE vs CDMA/EvDo and why the iPhone is not 3G.
The article points out the high-speed EvDo support will most likely increase the price of the iPhone, which is already too pricey even for a high-end smartphone.
EDGE is about .25Mbps fast, while EvDo with REV .O is 3.1 Mb/s. Cingular, however, will never support EvDo. The article also notes that EvDo has several classes and so does EDGE, in which case some EDGE class qualify as 3G.
WiFi better than EvDo and HSDPA
Future users of iPhone need not worry about data speeds when using some of the phone's Internet capabilities. At transfer speeds between 6.5 Mb/sec and 20 Mb/sec, the iPhone's onboard WiFi can handle web browsing better than EvDo, HSUPA and HSDPA.
WiFi is also much cheaper than using alternative services from wireless networks. In effect, Cingular loses revenue in its deal with Apple because of the iPhone's WiFi capabilities, which of course translates to lower cellphone bills.
Update: The iPhone got 3G but the price actually dropped.