In my experience, you'll get the best deal by going to a local cell phone store. No, not the local Cingular store, but the mom and pop places. I usually head down to chinatown. Not only will you get a free phone with cash back with 1-year service (usually after mail-in rebate or from the store after 6-months of use), you can get your phone unlocked. The unlocked phone can then be used internationally or when you switch to Tmobile (since ATT is now Cingular, that's no longer an option), which will save you more $$ later on.
EDIT: In addition, Cingular service in NYC is decent, but I HATE their customer service. They are closed on Sundays and have 30 minute wait times. Also, they charge you one month's bill in advance, so when you cancel Cingular, you have to remember that you already paid for the upcoming billing cycle.
