I live in the DC area. I've had both Sprint and ATT/Cingular Service. Edge is "ok" in terms of speeds. One minute I get like 150kps and the next time I'm only getting 40kps. It's not that reliable. Don't buy the iPhone if your main mode of browsing is going to be through EDGE. Edge is good for basic emailing, checking quotes, weather. If you plan on surfing the internet via EDGE instead of Wi-Fi, you are going to be disappointed. On the other hand, I've tried Sprint's HTC Mogul and wasn't impressed with "SprintSpeed" either and that's supposed to be high speed. It's feels slightly faster than EDGE in terms of loading similiar web pages. Maybe's it was the Mogul's browser, maybe opera mini would have been better.
In terms of Cell Phone reception, I feel ATT's coverage is much better than Sprint's in the DC area. I had Sprint for 7 years but the majority of the time I lived in Kansas City and Sprint was the king there because I was only a mile away from their national headquarters. I couldn't stand Sprint's dropped calls especially on the Tyson's side of the Capital Beltway. If you live in the city, than Sprint's coverage was excellent.
Verizon's coverage is probably the best in the DC area. Verizon does have Metro coverage.....that's because they (the former Bell Atlantic) had an exclusive contract with METRO for their towers down there. You shouldn't talk on the Metro anyways🙂