Okay .. here is what I find out from newsgroups, howardfourm, and the
cingular store:

1) Eventhough Cingular has bought AT&T the merger is not complete when
it comes to infrastructure consolidation.

2) AT&T's SIMs allow you to search for available networks, and select
other available networks. Users have the option to manually select the
network they want (phone will always try to use this network; no
connetion if the selected NW is absent) or set it to 'automatic' (the
phone decides the network to use based on signal strength).

3) The above setup has been great for bayarea because South bay (San
Jose, Santa clara) etc have pretty good Cingular coverage while
Peninsula and SF have good AT&T coverage .. and i could select the
carrier i want depending on the area i am  in.

4) Eventhough Cingular gives out unlocked phones, they disable the GSM
network select option in their SIMs. As a result I don't have a way to
force my T637 to use AT&T instead of Cingular.

5) The drawback with the above system is this: If there is a faint
Cingular signal in a particular area, the phone is going to try to use
the Cingular signal instead of a more strong ATT signal. This is
exactly whats happening in my apartment where ATT is good, but the SIM
doesn't allow the use of ATT. As a result i get crappy connection in
my apartment.

6) As far as I know there is no way to program the Cingular SIM to
accept other networks. If there is one, I would gladly take the
suggestion.

7) There is one trick I could do .. to insert an ATT SIM to my
cingular T637 phone, select the network to ATT, set the search mode to
Manual instead of automatic. Now if i put back the Cingular SIM the
phone would remember the ATT network selection and remain in ATT. But
I don't know on what conditions this will revert back to Cingular
network and whether it would go back to ATT network in a place where
ATT signal is stronger than Cingular.

8) All this would be a non-issue in an years time .. when ATT and
Cingular start to look and behave the same. But till that time i don't
want to be forced to use Cingular when i know another carrier who has
a stornger presence and free roaming agreement is available for me to
use. So i am thinking of cancelling Cingular service.

Thus far I've only tried using Cingular CSD while on the ATTWS network, and
that isn't working.  Later today I plan to try Cingular CSD while roaming on
the Cingular network (which doesn't cover my current location).

'Fraid not -- I tested it today, and it looks like Cingular has blocked CSD
connections for roamers.  [sigh]