Someone in a comment asked whetehr you could unlock the Nokia 6030 from T-Mobile and put it onto Cingular. So far, all reports I've heard give a resounding "Yes". You should just be able to unlock it with a normal unlock code generator, but I would do a Google search just to make sure.
And yes, you can use a postpaid Cingular SIM in any prepaid Cingular phone, or t'other way around, with no modification of any sort.
Hope this helps anyone having questions!