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I don't know answers to all your questions...but can answer the last one.
3. Cingular uses GSM technology, and so its phone uses SIM cards. Unlocked GSM phones will work on Cingular networks. The reason why you may not have heard about Cingular is because T-Mobile offers best data rates + plans (what is a Smartphone without data) and AT&T carries Smartphones, but Cingular has none of the two qualities.
Cingular till a couple of years ago was mostly TDMA based, even now I believe 10% or so of its network is TDMA based, which is not compatible with GSM.
Another thing to keep in mind is that Cingular is focusing on 850Mhz band for its GSM networks, and the current crop of smartphones don't support 850. MPx220 is the only announced (or unannounced) phone with 850Mhz support.
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