The Cingular/ATTWS merger HAS not occurred.
From a GSM network perspective in the bay area. The Cingular network is the old Pac Bell GSM network. T-Mobile rents access on it and ATTWS has its own network.
Just before the merger, ATTWS and Cingular signed cross-roaming agreements to get better national GSM coverage. So Cingular customers can use the ATTWS network and vice-versa.
Now here's the catch, Cingular just agreed to sell the CA/NV GSM network to T-Mobile to raise enough money for the ATTWS acquistion.
So for now, as a cingular customer you have both Cingular + ATTWS network coverage. As an ATTWS customer, same. As T-Mobile just the cingular network.
Post merger (probably about 1/1/2005 unless moved up), Cingular (including ATTWS) customers will ONLY have the ATTWS network in CA/NV. T-Mobile customers will be on the former cingular network.