iPhone: How good with current data transfer rate.

• Cingular's Edge network provides nationwide coverage at average data speeds between 75 Kbps and 135 Kbps, according to Cingular's Web site.
• iPhone's unique Visual Voice Mail was co-developed by Apple and Cingular, This requires hihg data speed to operate.
  
• Similarly the Wi-Fi capability, which can deliver high-speed data access requires high speed data transfer.
  
• Cingular and other GSM-based service providers are working on technologies such as High-Speed Downlink Access (HSDPA) to boost their network data speeds , although they aren't expected to be available in time for the iPhone rollout.

So In the begenning it may not be possible to use all the features of iPhone in low data speed.
Source: http://www.informationweek.com