Hi Dave,
This will only work if you had enabled 'Find my iPhone' on the device.
Then all you need to do is go to
https://www.icloud.com, login with your iCloud account credentials and click 'Find my iPhone'. You could also use another iDevice with 'Find my iPhone' installed to track the lost phone.
But all this has to happen quick before the thief re-install the Firmware on the iPhone or disables the network. Then the iPhone can't be found.
Always enable a Passcode Lock on your iPhone as this delays the process of disabling and deleting your personal settings and data.
While I think Apple has done well with the 'Find my iPhone' feature, I believe they could have gone further and allowed fro a way to find the iPhone by IMEI even if the account settings it is tied to are changed.