I took a look at eBay and you can buy unlocked ones for $540 or so with shipping included. The cost for canceling a recent contact with cingular is $150, so you're cost from amazon would be $480, and you'd have the hassle of canceling contracts and waiting for mail in rebates. If it was me, I'd spend the extra $60 and just buy an unlocked phone on eBay and not have to wait for any rebates.
Actually, I'd probably buy the slightly used ones for $425 on eBay to save another hundred dollars