I recently traded in my ATT Wireless SMT5600 for the Cingular model though Cingular's tech support/Replacement Center.
These are the differences I found checking out the two phone's ROM and WMP 10 options.
Items in green are higher numbers - I'm not sure if higher is always better, but it could. :wink:
I have to send my ATTW phone back so there won't be any time to do any more checking out the versions on the phone, but you can feel free to ask me any questions you may have about the 'new', Cingular model.