I don't know about Washington DC, but in NW Indiana, Cingular's coverage has always been good. People I speak to don't like Cingular's customer service. My friend is in the process of cancelling his service since Cingular is giving him a hassle with his AT&T contract, and he's on TDMA service, and GSM only works in heavily populated parts of NW Indiana. Most of rural Indiana isn't even covered by GSM. If you decide to go with Cingular, expect poor customer service from them. My friend is one of those dissatisied with Cingular, along with my mom & sister, plus a cousin. Cingular didn't meet my needs when they were Cellular One, and don't meet my needs today.
I have Verizon right now and they pretty much have the best coverage in this area (Wash, DC Metro), but as you all know, they don't always have the latest or greatest phones. They have the Treo 600, but I would really like to get the 650. Is Cingular any better now that they have merged with At&T?