It's my understanding CDMA networks do not support the "long SMS" method of concatenation, which is most of the US cell carriers, while of course most of the rest of the world is on GSM networks. US GSM carriers like Cingular/AT&T DO support long SMS, but only within their network (Cingular->Cingular), if the message goes outside their network (Cingular->TMobile) then it loses the special instructions that allow for the concatenation.
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