In Northern California, even though you might use a T-Mo SIM card in your phone, it says Cingular. T-Mo uses their system.
Question: I love the feel of the updated BlackBerry (Black, 8700g and 8703e) a lot more than the old (Silver, 8700c from Cingular) which I've used at least a year.
Will the same transfer work with BlackBerry? In other words, if I buy the 8700g from T-Mo, can I use the 8700c Cingular SIM card? Or do I need to use a T-Mo one?

You'll need to have the 8700g unlocked or buy an unlocked one.

I just called and checked and T-Mo said all the phones they sell are locked for a minimum of ninety days. Thanks anyway, John.

There are lots of unlocking services out there that can unlock BB's. Also, they can be purchased on eBay unlocked. Beware and shop carefully on eBay, however.