I would contact Cingular. I know of a guy who tracked down his friend's sidekick on Tmobile. I am not sure that Cingular works the same way, but when she bought a new one, and they put her sim in, it downloaded data from the t-mobile server and there were AIM profiles and pictures of the new guys who had her phone . . . . He took it to the internet, and the short story is, they got busted . . .
An interesting read . . .
http://www.evanwashere.com/StolenSidekick/